Seems like on balance, single guys are more likely to be available, and less likely to have to cancel at short notice. They're more likely to be taking care of their appearance.
Married guys are less likely to be clingy, or to try to escalate the relationship part beyond a certain point. STIs are somewhat less of a worry, if your wife is the only other woman he's seeing (assuming his wife is not also sleeping around!)
Ethically, a reasonable argument can be made that the moral transgression is squarely on the person sneaking around, breaching trust, breaking their marriage vows, etc. It does seem fair to ask yourselves what that says about his character. But other peoples' inner lives are unknowable, and sometimes they have their justifications.
Leggysandy doesn't knowingly participate in cheating. If she felt differently, I would be okay with that, but I prefer her approach of trying to keep everything and everyone honest and above-board. Even if it means that sometimes a great candidate gets passed over
