Always had au pairs an if they had a boyfriend for 3+ months let them have them over whenever. As long as they keep to themselves and not sit around the living room while DH and I are spending family time with DCs I don't really care.
Ive always liked their partners and don't see a problem in them coming every few weeks. We had one come every weekend and that meant they never had any spare cash to go out, so they were always around for Saturday night babysitting - they'd even play with kids on Sunday mornings so we could have a sleep-in. Bliss.
I know what you mean about homesickness. I wouldn't have a problem with a boyfriend from home coming once a month or so. Our ex-APs boyfriend moved to the UK with her...was around every weekend initially. Would'nt repeat.