I'd not become a new friend to a guy that didn't immediately introduce me to his partner. Mileage varies, obviously but becoming new friends with a member of the opposite sex is a fundamental relationship boundary cross that we as a couple, don't engage in.
and this is the very attitude by "some" people why we don't get involved with new relationship of the opposite sex. He's in a relationship... he's in a relationship, he's not open or available to get to know you more... anything while hes in a realationship is superficial and very, very likely on the down-low.
All, in our world, is not fair in love and war.