You only X because you Y. (with Y being true!)
Example: Paul only defends the drug decriminalization because he has never been aggressed by a drug addict.
Is this a fallacy? If not, how can we refute it during a debate?
This question highlights an important point --that identifying a specific fallacy is really only important in as much as it helps craft a response. With that said, I would see this as both an ad hominem attack against Paul (he doesn't have such-and-such an experience, therefore his argument must be bad) and as an appeal to authority (the arguments of those who have had this experience must be correct).
It's important to realize that the attacks against you would not be fallacies in two specific cases --if you really have no other argument in favor of your position than just your positive personal experiences (then the argument against you is correct, not fallacious) or if the person who had the negative experience really is the correct authority AND would disagree with you (for instance, if it could be shown that having that negative experience is a common or reasonably expected result of your position). Thus, your defense lies in debunking those two possibilities:
I think this question has a context specific answer. For example, you are arguing for decriminalization and state "X only because Y," regardless of the truth value of Y the statement is meant to be derogatory in order to support your claim. Therefore it could be considered an ad hominem against Paul, or if Paul was to speak to the person you are stating "X only because of Y" to, then it could be considered "poisoning the well."
Regardless of being a fallacy in some cases, this is not good reasoning or logical justification for any argument, and is more along the lines of a debate tactic.
This seems to be more of an accusation, or suggestion of a hypothesis than a statement of fact.
There are three ways this can be untrue: "you only X because you Y" eg "you only rap because you're young".
can be false if .. 1) You're not young and yet you rap 2) There's no demonstrable link between rapping and being young 3) The word "Only" - You might rap, but it could be for other reasons - this overlaps with 2) a bit though.
Note that the statement could be true if I don't rap. If I'm not young, then perhaps I don't rap, and that could be the reason why. [Actually: it's because I'm rubbish at it & prefer singing lol]
In everyday terms, accusations like this are used to establish a connection which raises questionable morals, such as a politician only voting for a particular act because they stand to profit. So they might refute this with:
1) I didn't vote that way. 2) I am objective enough to vote for the greater good without letting my own preferences get in the way. In this case, they aligned to the same thing, but may not next time. 3) I voted that way because I stand to make a profit, AND my wife told me to.