It depends what you mean by "dating". If it includes sex (See, I'm of an older generation: this is not a given for me. I often dated without sex.), and you enjoyed that sex, and continued to be attracted to transmen, then you would not be straight, you'd be bisexual.
I think what sometimes happens though, is people are sometimes attracted to certain individuals who are not normally in the group that they are interested in. And it's enough for people to say to themselves, well, sure, I would be ok with that more generally. They are thinking that if they are attracted to one transwoman, who maybe passes rather well, not being attracted to others is just like not being attracted to any individual. Especially if the attraction is more abstract, someone on tv say.
It might be more the case with trans men, too, because many of them if they have transitioned for long can really pass totally, at least with clothes on. If you are getting al the visual signals that say "man" then the sexual attraction part of your brain may not care about much else.