On my experience (cis male, 35 yo, married with a bi cis female): my attraction towards different genders do oscillates and sometimes I'm very focused on a single gender. But it's not that I don't feel attracted to other genders. It's that most spontaneous sexual thoughts tend to come with a specific gender in mind.
But even when most of my spontaneous sexual thoughts are directed to males, I'm still very (VERY) attracted to my wife, for example. And would be sexually interested in people with other genders as well if that would happen. It barely affects that.
Think about it like this: sometimes if feels like situation if I was single and went at a party, maybe I would unconsciously only approach females depending. But that wouldn't affect my interest on any males or females (cis or trans) if any of them eventually approached me. I'm still attracted to them, it's just that it takes a little longer to notice, but anyone pressing the right buttons would be attractive. Others times if feels like I would spontaneously think sexually about someone regardless of gender.
I don't know how that works, but it's the way I feel. I can spend days without noticing any males around me. And then suddenly I am on the subway and "Wow, that guy is hot... That one too... That one is not that hot, but looks like a good conversation..." aaand that's it, I'm gay again. This happens with females too.
That said, I think there's very little evidence that attraction towards same gender can fade with time. Since I'm not absolutely sure, I won't say "no, it's impossible", but if it happens at all it's pretty rare.
What does happen is that people force themselves to conform after "going through a phase in adolescence" because that's what is expected from them. See, I'm a cis guy married to a cis girl, and I can say that it's not that difficult to decide to live that life and stay that way. I will continue to be in a monogamous for the foreseeable future and that's what a lot of people do (the only difference being that I decided not to repress those feelings).
But this is forced, self conscious repression of what you feel. Nothing "naturally faded".