What do you think of me having an affair?
Is there already somebody else?
Someone else? Yes, sure there is someone else
Relationships are difficult but having somebody else in the relationship makes things more complicated. It could lead to your relationship ending.
It's just sex, it has very little to do with the relationship, it's great because I'm getting both.
But that is wrong if your partner is not okay with that. Sex is cheating and you risk giving your partner a disease without them knowing it.