Rational just means that it's based on reason or a belief could be rational if it's grounded in strong evidence.
So being "rational" isn't really this seal of approval that people nowadays try to make it out to be and could actually be a very low bar to pass.
The thing is, life is multi-dimensional chess with and against countless of players so whatever move you make could likely be reasoned for or against in some way or another and thus be called rational. So people love to throw the word "irrational" around if they don't immediately see the reason behind an action or if it doesn't fit their own line of reasoning or a particular technical definition of "rational".
So idk classic examples might be these game theory ideas, where people give out some setting and a heuristic or goal function and if your move maximizes the game function then it's "rational" and if it doesn't it's "irrational". And then they find games like the prisoners dilemma or a dollar auction and taking "rational" steps ends up with a the worst possible outcome.
So in other words it's not rational to be "rational".
(Or you have to give up the idea that rational means smart or optimal or you need to give up the idea that rational is a single action or you need to give up the idea that what you considered "rational" is actually rational or ...)
The only way in which you could be truly irrational is if you'd go for complete randomness, but I'm not even sure that you could do that even if you wanted to. Like you could act on whims and listen to your emotions, but then again your body is the result of thousands of years of evolution that created mechanisms to keep themselves and in consequence, to keep you alive so being alert and in touch with ones body might not even be the worst approach, but actually follow some reason. Even basing your actions on a coin flip might be a rational approach, like suppose you have an enemy that tries to predict your next move to get you, so if your action pattern is erratic they have a hard time doing that, not to mention that you might still hit a goal by probabilistic approaches.
So without further defining a technical definition of rationality, that bar is so low that you could almost by default say: Yeah it's probably based on some reason.
Though that doesn't mean that it's an ethical, moral, optimal or otherwise useful reason.
Also "pride" seems to cover a whole spectrum of possible behaviors and emotions. From confidence in one's abilities, to being content with who you are, gaining confidence and self-worth from one's achievements, to asserting a social status, praising oneself beyond any reasonable relations, chauvinism, etc.
And it has apparently been conjectured that this might have evolutionary benefits, like in showing pride and asserting a social status might secure better positions in a group and thus better food and mating abilities. At the same time it could also mean that others are worse off and could thus also create conflict within a group, so it's not unreasonable that it's both treated as virtue and vice depending on what end of the stick you fine yourself.
So as others have pointed out this is somewhat of a fallacy of equivocation, in that "pride" in the context of the LGBTQ movement is not really about some chauvinism or showing that you're better or more deserving than the rest. On the contrary coming out as Queer was and in some places unfortunately still is more of a way to ruin your social status.
So "pride" in that context is, again shamelessly stealing from good answers, about showing that your fine with yourself in a context in which people tell you that you're doing something shameful.
Which serves a purpose both for those participating which might gain a confidence boost, but also for those still being afraid to come out. Like while it's easy for a cis hetero man to tell people that things have become normal and that there's no huge risk in coming out today, usually you become aware of your sexuality in your teenage years and that's also the time where everything is incredibly embarrassing, where you think everyone is looking at you and where you really don't want to stick out. Until you do that anyway, adopt a healthy "don't give a shit"-attitude and find out that most people don't give a shit either, because they're busy going through similar shit (not always but for many that's just what "growing up" is). So easing that process and making sure that this leap of faith of being yourself ends up being more of a jump from a curb and not a skyscraper might still be important.
Also as long as people feel the need to make sexuality in general and queer sexuality in particular, to be something weird that you don't talk about, it absolutely makes sense to actually do that and to confront fears with reality. Especially if reality is not actually scary or harmful in the first place.
Now for people with majority preferences that still is plenty scary, you also have fear of rejection, of showing yourself vulnerable, of that maybe being exploited, whether you should tell anyone about it or keep it a secret, before again you go with fuck what the world thinks about it, I'm (hopefully we) are find with that and that is (in this situation) all that matters.
So this additional anxiety, that is not related to your partner or yourself, but just to the general group from which you've selected your partner isn't really an issue, that needs to be addressed, because so far we've more of a heteronormativity anyway, so if you don't state your sexual preferences it's assumed you're hetero.
So displaying a heterosexual pride usually comes with a different vibe in the sense of "why are you proud of something that is neither and achievement of your own that you draw confidence from nor a resistance to being shamed for something and an act of standing up for yourself (together with others)?". It's also not related to the individual fear, which you might address with vandalism (you know massacring trees or spray paint a heart with names for the world to see).
So what is it about? And well usually it ends up being more of a sign of the other meaning of "pride" meaning some sort of chauvinism, that wants to assert a privileged social status and that uses straight as "anti-queer", so rather than showing that you're fine with yourself you assert that others should not be fine with themselves.
Now if you are a homophobic asshole who wants people to feel bad about themselves than that might still be "rational"... Morality, ethics, basic human decency and whatnot would want to have a talk with you though. Also if you read rational as scoring the best outcome for yourself, than no getting into petty fights about other people's preferences that don't effect you and pushing people into secrecy is not likely to produce a more livable environment.