My wife's dwindling interest in sex, is what finally got me into bi sex.  As long as I was satisfied by her, it wasn't an issue. But like most people, I wanted sex with someone that wanted sex with me. The easiest way to get that was to let guys suck me.  It didn't take long before I found my first cock to suck. 
Going from feeder to cocksucker isn't inevitable, but at least in my case it was a natural progression. Best of luck.