What is it with straight men and wanting to be a 3rd wheel in a committed relationship?

Why do they, (even when they know we are committed to one another and would never be interested in him), seem to be so happy following us around, letting us cook for them, and sort of attempting to be a "third wheel" so to speak, in our relationship? We love our straight male friends, but it seems like all of our single straight male friends want us to "mother" them or something. It's almost as though they want in our relationship, even though it's not possible. Their interactions with us are perfectly platonic and non-threatening, but why is it that they keep trying to become more intimate with us, to the point of almost inviting themselves out on our romantic dates? Why this comfort with being a little too intimately involved in someone else's relationship? I don't see straight men trailing heteroual couples...can someone explain??