When you are serious about someone the time you spend on other friendships naturally recedes as the primary relationship progresses.
There is no need to "declare a boundary", rather you simply sit back and observe what happens.
I have not dropped any of my male friends. In fact I take one of them to his chemo treatments if he needs a ride and my husband and I help him if/when he asks.
But all of my friendships including with my male friends have receded in light of my marriage. This is normal. We are still buddies, I just naturally interact with them less and my single guy friends know that I'm not going out with them to run around now like we did before. I don't think a married woman should do that....but I don't need my husband to tell me so. Duh. Just last weekend my husband got a wild hair to go to a bar my male friends frequent (we were out nearby), so we did.
Two of my buddies were there. One had a gorgeous new girlfriend (she was cool, I quite liked her). They were both out with me the night I met my husband. My husband went right up & said hello & we all hung out for a bit. No Big Deal.
As Duke said, you observe & let a woman show you who she is. Setting these boundaries like a jealous 13 year old is NOT So Suave.