I'm bisexual and I've never cheated on anyone, but merely find both men and women attractive. Although some bisexuals may be promiscuous, just as some gay and straight people are, others are not.
In my own case, I usually enjoy longer term relationships with men and then brief, but exciting sexual encounters with women before moving on to another relationship with a man. I accept this can be extremely frustrating and irritating at times, but I wouldn't cheat just because the opportunity presented itself.
Some bi-sexual women prefer longer relationships with women and enjoy brief dalliances with men between relationships, whilst others have relationships of equal length with both sexes.
I'm certain there are others who merely flit between the sexes in a manner equivalent to men and women who have numerous partners, but I think that is more a reflection on your personality type than your sexuality.
In answer to your question, I'm not sure it is a matter of fashionability, more the easier acceptance of various sexualities, particularly among women in this day and age.
Sadly, there is a degree of discrimination against bisexuals from small section of the gay community, but I see that as their problem, not mine.