Is it the overtly sexual nature of this that you find shocking? And - intensely personal question incoming, feel free to not answer - has he ever shared this particular kink with you personally, and is it the kind of thing you'd be down with?
As my husband and I have explored our poly natures together, we have discovered aspects of one another's sexuality that we hadn't discussed previously because we were afraid that sharing them would either make the other feel uncomfortable or inadequate in some way. So while he knew that I had bi tendencies, he had worked very hard to not display his overtly because he was scared that I would somehow see him as less manly (gotta love that cultural lie that bisexual women are always epic and awesome, but bisexual men are always effeminate).
When he finally "manned up" and told me about his experiences with men, I was thrilled, both because I'm always an advocate of honesty and because it opened a door for things I had always wanted to ask for but hadn't for fear of making HIM feel inadequate. But I was also hurt, because some of those experiences happened while we were dating/living together (but stopped when we got married).
Finally though, through reading this site and lots of communication with the husband, I realized that he hadn't told me because he thought he was doing something that would horrify me. Men in happy committed heterosexual relationships just don't kiss other men - you know, in the "real world."
We've talked about it a lot, and at this point I am excited at the idea that he now has the freedom to be who he is, while still being my wonderful loving husband.
If this is the first you're hearing about his oral thing, I'd have to ask myself why. It may be that the random quality of finding women online is what does it for him, which frankly could be problematic for me and my lifestyle and would require a lot of discussion for me to be comfortable with. It may be, though, that he's concerned that telling you about his "kink" will put you off, like my husband was. Either way, further discussion seems called for! Good luck with this!