What if I couldn't get hard with a married woman?
That sounds complicated. Why was being unable to get hard a problem?
Because's she's awesome and we have awesome sex.
You shouldn't have sex with someone who is married. You are being selfish by indulging your own sexual desires while ruining the marriage they have. Their partner would be crushed if they found out what was going on.
Why shouldn't I? The guy is not in the picture and the wife knows what she's doing.
Even if the husband is not there right now, he still exists and by having sex with a married woman, you are being a homewrecker. Getting in the middle of somebodies marriage is bad and frowned upon by others.