First of all, welcome.
What you said is interesting, because as a trans man myself, I would really prefer to date a straight girl - bisexual would be OK too, but I wouldn't want to be with a lesbian because, well, they like women and I am not a woman. I'd feel better knowing that the person I was with was attracted to cisgender men, because I'd feel like she saw me as a guy too. So it's kind of interesting that many trans men you know are with bi/lesbian women. Maybe your theory is right, that they are maintaining the relationships they had from when they were "female."
As for the family thing, I think the easiest way to explain it would be that you like trans men because even though they have different parts (when they are pre-op) they are still men in terms of who they are. Another thing, which I'm sure you probably know, is that after long enough on hormones and such, almost all trans men become physically indistinguishable from cisgender men (at least with clothes on). I know for me, when I see a beautiful woman, I don't think "oooh, she has breasts and a vagina under her clothes" (lol, well, sometimes I think about those things but you know). So if you meet a guy, and he's handsome, you don't immediately think about his penis, you think about the whole package (lol) as it were. So, if it turns out the guy you like doesn't yet have his penis - well, maybe that's not such a big deal because you find the whole guy to be attractive enough that you're not going to get hung (oh, the puns, they kill me) up on such a minor detail.