He's stating his opinion.
Basically, the information and research we have to-date tells us that sexual orientation is most likely a combination of hard-wiring, very early influences (like most things sexual), life experience and yes, choice. And certainly, no matter our orientation, we do choose our partners and choose what sex we do or don't engage in.
Ultimately, if orientation was 100% choice, that would mean that heterosexuals could just as easily choose to be homosexual. We don't even need to look at all the data to know that's pretty unfounded. Heck, even working for years in sexuality, all I need to do is listen to my single, straight male friends in their thirties who go dateless waiting for the right female partners, bemoaning that they aren't queer, and can't just opt in, because in Minneapolis there are often more available men.
Really though, even though we do know pretty clearly it's not 100% choice, or likely anything close, it really wouldn't matter much if it was, would it? Because choosing a given partner -- even let's say, between one woman and another -- doesn't make the one you chose any less meaningful because you chose her, it likely makes her more so. I'm sure your father likely chose your mother as well, rather than simply accepting whichever woman came along first.
So, no, research and data doesn't support what your father is purporting. But focusing on that may be a waste of energy and real communication: it might be far more productive and important to talk about why you *chose* this particular person to be in love with, not by virtue of their gender, but by virtue of who she is and how you feel about her.