Let me get this straight...You're looking for another relationship along with the one involving a married man, who isn't telling his wife about his affair with you? I'm cringing at the train wreck to come!
And if you're looking for a "filler" boyfriend to occupy your time when your affairing boyfriend is unavailable, don't you think that's a bit...unfair? I mean what you're doing is giving this secondary boyfriend a job which may be at times rather upsetting to him because he may want to be with you, but he can't because you're automatically taking time with your first boyfriend...who is having an affair with his wife with you. It's enough to make anyone feel like their time is being objectified.
Sorry if I sound snarky, but this just doesn't look at like a sturdy foundation in my opinion.