One useful principle which ethicists appeal to (especially those in deontological traditions) is roughly as follows:
Ought implies Can: One ought to do something only if they can in fact do that (even if they might not actually do it).
The idea is that nobody is at fault for not unilaterally solving world hunger since that's simply not something any of us is capable of. I suspect that the same might apply to 'types', that is, it isn't really practical to force ourselves to find things physically attractive which we don't already find physically attractive. This depends on the empirical premise that our physical type is not very malleable, which might not be true. Another possible formulation would be on a version of consent where absolutely no reason is a bad reason not to seek a relationship with someone.
The alternative might be formulated as follows:
Implicit Racism: While someone may seemingly have an (innocuous) physical type on the basis of (for this example) race, their 'type' might in fact be just a subconscious (or even conscious but secret) racism, and this racism is the problem rather than the physical preference itself, even though the physical preference wouldn't exist without the racism (e.g. this is pretty overt in most WMAF pornography).
It seems easy enough to make a moral case that this kind of 'type' is morally blameworthy. The real question is empirical, that being whether this is in fact the source of people's 'types'.
Another way of putting it might be on consequentialist grounds:
Combatting Systematic Racism: Someone considered to be in an unattractive racial category may have disproportionate difficulties finding a partner (since our type is likely shared by people in our community), whereas we are able to find love with members of this racial category despite them being 'not our type'. So the benefits overall are positive to going outside our type and this means we ought to do so.
This relies on the empirical premise that dating someone we consider less attractive is not so bad, and that the conditions faced by individuals who are less conventionally attractive are worse.
To an extent the empirical parts of all of these might be true, in which case we need to establish which 'rules' take priority, whether we need to try to make our type malleable despite the challenge of that due to racial or consequential considerations mattering more than the deontic principle.
