need help!

This may sound silly but my boyfriend asked me if he can have another female sleep at his house, (my parents will not let us sleep at each others house) and for some reason I don't like the idea,like I trust him an all but I don't know why the idea doesn't sit right with me. He says it gets lonely when I'm not there because we can only see each other on weekends,and I really have no clue what to say to him..
Please help me make a choice 
(Am I marking a big deal out of nothing)