what do you mean, your wife having a boyfriend on the side? Where there is more than just sex? I actually like the idea. My wife has had sex with about 8 guys over the past couple of years, but only a couple of repeats. I think a bf would work better for her. I know it sounds strange.
My wife sees two guys regularly. The comfort level for her being the main reason for that. No actual dates though. One of our rules in the very beginning was no real dates, just sex. This has worked out rather well so far.
Personally I enjoy both categories.
Nothing like having a longer term bull for me. But also the thrill of a one night stand is attractive. Makes me feel slutty when a bull I don't know well pumps and dumps.