Secretly Bi I could maybe see, but why secretly? Again-- several hundred years in the future, with a very liberal bent on human society's trajectory. In just 50 or so years we have gone from public lynching of gay people to gay marriage. In the several hundred years we are talking here, being openly bi would be as ordinary as being openly straight, or openly gay. Just something a person is. There is likely no reason to keep it a secret, because there is nothing to be ashamed of or worried about.
Yet there is also no reason to bring it up, either, because it's totally normal, any more than you'd specifically bring up being straight.
The topic of childhood sweethearts comes up in game 1. You can ask Kaidan about his childhood, which he expresses as being rather unpleasant. You can ask if there was anything that made it more bearable, and kaidan will reveal information about said sweetheart. Normally, a bisexual person will not exclude a gender from such a conversation. (I have had several such conversations, and it has never been so.)
I'm not totally sure what you mean by "exclude a gender". If you mean "not explicitly mention the gender of people that person was attracted to", that's the exact opposite of my experience - that is, I've never heard
anyone of any sexuality specifically mention that a particular childhood sweetheart (say) was a boy or a girl, outside of the vocabulary limitations of the language (pronouns and "boy/girlfriend" in English, noun gender in many languages, etc.) If, on the contrary, you meant "only mention people of a particular gender", that's somehow even
further from my experience; many bisexual people report having had only heterosexual childhood relationships or crushes. A few even report having had only homosexual ones. I can't actually think of anyone I know personally who definitely
didn't, although I don't talk about childhood sweethearts with people much.
Casual conversation is a very good barometer to determine a person's innate sexuality, because they will spend lots of time discussing people they liked, and patterns in attraction will naturally appear, even if they dont mean to. Kaidan exhibits a clearly heterosexual pattern, with a preference for tough girls.
Yeah, no, I know people your "pattern" theory here would explicitly fail with in exactly the same way. So it's not odd at all to me. (Also, uh, some people just don't spend a lot of time discussing people they liked either, you know.)
If Kaidan is secretly bi, why be secretive about it? If Kaidan had the hots for shepherd since disc 1, his dialog would have hinted at that unless he was conditioning himself to not reveal anything, which again-- he was failing miserably at.
Professionalism? Not thinking Shepard would be interested in return? Simply not developing feelings about Shepard until later? Any combination of the three? I've never tried to suggest that Kaidan was hiding anything on purpose - I've never played the game and have no idea about the backstory - but the simple pattern of behaviour you describe is 100% plausibly realistic.