I am married (over 43 years), had an experience with another teenage boy when I was 13, but lived a pretty normal heterosexual life since, until fairly recently.  Sex with the wife has become less and less frequent and not really being interested in an affair I turned my attention on-line for getting off.  In so doing, I started becoming bi-curious.  Over time the curiosity increased and one of my fantasies was cuckold/stag-vixen related. 
Imagining watching my wife get fucked by other men is a turn on.  Then thinking about participating in that scenario was also a turn on.  Those thoughts led to imagining that I was stroking that other man and even sucking his cock in front of my wife.
I started connecting on-line with other married guys that were in similar situations.  What I found was other guys that decided instead of going out and finding another woman to fuck, why not just play with another guy.  Soon I connected in person with another guy and since have sucked three or four guys and stroked another four or five.  So, I guess that would make me bi, but I am not big on labels.
I know that at this point I am not interested in anything but stroking and sucking.