as a trans man myself i think it completely depends on how the transmasc person in question identifies?
like, you say "transmasc," but that could mean anything from a 100% male binary trans guy to a nonbinary butch lesbian. transmasc is just an umbrella label for people who are trans and identify with masculinity; so with a broad question like that it's sorta hard to answer.
if we're talking a binary trans guy dating a straight girl then like, yeah obviously that's a straight relationship. but if we're talking a transmasc genderqueer person dating a straight girl, it's sort of up to how that person identifies, in terms of the relationships "straightness." in my opinion, at least. if that person identifies as sapphic then dating a girl isn't straight & thats something to bring up to their partner -- if they view their relationship as a straight relationship, though, then that makes it a straight relationship. or they could not really be thinking about it in any strict terms like that at all and just be vibing.
ultimately gender and sexuality are very confusing but like bottom line, tl;dr, etc: in my opinion, it depends on how the people involved view their relationship. i know for me personally if i was dating a straight woman i would view that as a "straight" relationship & i would hope she would too, but everyone identifies differently & transmasc is a very broad label.