But seriously, since you did ask...
I wouldn't jump to conclusions just yet. How long have you been married?
I have known two types of leos...there's the flirty yet loyal kind...and then there's the "cassanovas."
The first is just a serial flirter - but never cheats. The second is a straight-up manwhore.
It is possible, if you've been married awhile, that he may just be bored and looking to see if he still has "it."
He may not even be looking for anything questionable, but rather wants to feel like he's still desired and hasn't lost his game.
Ego is a big thing with leos, afterall.
Hope this helps.