My hubby hasn't gotten a hard on that I know of, I'm pretty sure he would have told me, I'm into that kinda thing. I know he has gotten tingly or maybe had some movement.
I think it's normal, I like the thought of him with another woman in fantasy's only. so sometimes I'll get him to give details of his sexy thoughts.
I think having sex with another person other then your significant other is something we all do, Woman and men. I do it in passing sometimes I don't even realize I'm doing it till it's all ready done, so I think it's like that for guys to, maybe a little more so being they have more Testosterone.