Maybe you could both go together? Don't make her feel that she can't do the same things with him as she can with you. He's single and has not much to lose. Your relationship could be at stake for her.
Otherwise, it's bad enough that the guy is living in close quarters already. If feelings aren't there already, it's very likely that the temptation is there to start something. I don't think it is at all wrong to have a red flag up in your mind over this situation. All because most people seem to have relaxed their standards over time doesn't make the situations any more right or decrease your rights as a husband. You might call it "old-fashioned" but it's more like "proper etiquette" that people just don't practice anymore, even though they should.
Personally, I wouldn't even go to a bar alone with my brother-in-law or any other male than my husband (well a brother would be ok if I had one lol).
Give her some other options to have a good time with you....and try to get another place for the guy to stay. You don't need that tension so close by.
