I was also “that guy” to a woman I loved, Natalie, although in this case it was a matter of her accepting that she was lesbian rather than bi — although she had never had successful relationships with men (while having had several with women) she had thought that it was a case of not meeting “the right guy.” I was going to be that guy, but…
I knew she was a lesbian when I met her (we met through a mutual lesbian friend) so I did not see her as anything more than a friend… then a good friend… then gradually we became friends who kissed and cuddled and shared a bed when we traveled. But we didn’t actually have sex until after she asked me to marry her, and that (as she pointed out afterward when we talked about getting divorced) was a fatal mistake.

We’re still friends, she’s still wonderful, and my wife and I attended her wedding to a marvelous woman who has a startling number of personality similarities to me. 😁 (Although Carol might argue it’s the other way around. 😉 )