We have a 'rule' that if two are having sex the 3rd is always welcome to join if s/he wants.
^i love this rule
I don't think it's that big of a deal. I mean, it might be a big deal if you mean to ask them at all times, everyday to be done before you get home. That just seems controlly, if ya know what i mean. One one time or asking every so often is fine, lol. I think hearing people have sex with always make you awkward, no matter how mature you are. usually it turns me on, but sometimes even i get weirded out.