By two years into my first marriage I was fantasizing a fair amount about gay sex (mostly hot stories I'd read) during sex with my wife. By our late 20's it was pretty much a requirement to keep hard and perform properly. My porn collection evolved in the same way. Maybe I should say "our" collection, as I never hid it from my wife and we sometimes read and looked at it as part of foreplay. That eventually led to a fair amount of talk during sex about me sucking cock and her fucking other men, and things went on that way for the 2nd half of our 13-year marriage.
I was fortunate to have a wife that accepted this side of me as far as she did. She enjoyed the fantasies to some extent, but mostly it was about keeping me turned on and hard during fucking. We never got to the point of a bi threesome with another man, although (as I've described) she wasn't exactly high fidelity during our marriage. The fact that it was the height of the AIDS epidemic may have been the main reason we held back from experimenting.
I think that some guys are functionally bi in their teens and early twenties because they are horny enough to be turned on by almost everything. I remember getting hard reading the definition of "copulate" in the dictionary at age 13. But your interests can begin to narrow once the pure novelty of sex wears off and your hormones are not so overwhelming. The fact that it happened so quickly with me Is a pretty good indicator of where my real interests lie.
