I don’t know…see, I think an awful lot of male-female ‘friendships’ aren’t exactly that. It’s more like, the male has some sexual interest in the female, and hopes it will develop into something. Even if they’ve never dated, even if they’ve had a night or two that sort of tailed off – in the guy’s mind, at least, there’s still a ‘potential sex partner’ aspect to the relationship, and that is part of what makes him want to spend time with her. (It probably happens the other way, too, but more rarely. Basically because if the female wants sex with a male who considers her a friend, he is more likely to oblige her and then sex is on-going.)
Anyway, so things go along in this M/F friendship, casual time spent together, maybe semi-dates to movies, invitations to each other’s parties, hanging out after work and so forth.
But then something happens to make it clear the ‘potential sex partner’ portion of the relationship is over. It could be that she is now involved/engaged/married to someone else or it could be he who is now involved/engaged/married to someone else. Either way, a chunk of the motivation behind his pursuing this ‘friendship’ goes away. If your friendship was an 8, maybe now it’s a 6 or 5 in the overall scheme.
After that…it comes down to time, really. There are only so many hours a week, and once you subtract work/sleeping/general life maintenance, the free hours left are precious. Some you want to devote to your own solo hobbies and some are for activities that you mainly share with friends of your own gender and only some are left for spending time with/doing things with friends of the opposite gender.
It doesn’t strike me as at all strange that most of that ‘time with opposite gender’ gets spent with the new love – most certainly in the early times. Why wouldn’t you choose to spend that time with your beloved? You love her, you enjoy time with her, you are having sex with her…
Yeah, the demoted friend is going to miss you, but that’s life. She needs to find new friends to spend her time with, and hopefully the new friend will grow into a deeper relationship and then it can be her other ex-friends who will feel she’s not devoting enough time to them.