Because it is not for the purpose of procreation, but for enjoyment and bonding.
I didn't say that "only" "purpose" of every sexual act is procreation.
???
Enjoyment and bonding are an important part of sexual goodness. Let me wave my hands and say Hooray. It would be wrong to do something to deliberately impair pleasure-bonding, just as it would be wrong to do something to deliberately impair fertility.
God 's wisdom put pleasure and procreation together for a purpose. It's a great design. The more you know about it, the more awe-inspiring it is. God is a genius!