When my wife and I first started seeing each other it was strictly just sex, I am here dominant and she is my submissive (we were doing this before the 50 shades craze). I had collared her long before we got serious.
She has always seen me as a masculine dominant guy and that is what she prefers. I think she might be afraid that I will grow to enjoy sucking cock so much that I will want it to be a regular thing and it will effect our relationship. Honestly though, I would say I am bi but it would be more accurate to say that I am permanently bi-curious. I never want anyone other than my wife and I am not physically attracted to men at all or have any desire to ever be in a relationship with a man. Mentally I am pretty much straight (pretty much being the key words), it is just that maybe a few times a year I get a craving to suck a big beautiful cock. It is weird in a way because I go from having less than zero desire to suck cock the majority of the time to on a rare occasion feel like I am starving for a big delicious cock. "Thirsty" would be an accurate slang to describe me during those times. Then after a couple of days the feeling goes away and I have zero interest once again.
I will say this though, I have taken ecstasy with my wife and few times and even though we only invited a guy over that one time I want to suck cock and get my mouth fucked every time.
I will say this though, my wife likes X more than I do and on the rare occasion we do take some it is usually her idea, if she mentions wanting to do it again I am going to run the idea of inviting that same guy over again. He had a plump, but not too thick, beautiful cut 9 inch cock and I would love to get it in my mouth again.
(Can you tell I am thirsty as fuck for a big cock right now)