studio apartment with multiple tenants

[Eric Dubrule]

I have a situation where a current tenant wants his girlfriend to move in with him. He called me and said that half her stuff was moved and if it was ok for her to move in. I'm uncomfortable with this as I feel a studio is meant for 1 adult. Can I flat out tell him no or should I provide a reason? I feel even more uncomfortable with the situation because he has told me that the girlfriend is pregnant with his child. This doesn't influence my decision, but I'd like to know how other landlords, especially in Massachusetts, would handle this situation. thanks!