@Aretheus If you're talking romantically then sure it can be uncommon, since most people feel possessive in that kind of thing.
But clearly Maid-san likes sex as recreation, so Sometimes 1v1 is not satisfying enough when you've done it 1 on 1 for so long. Hell, as dude, sometimes I had the thought "Maybe it wouldn't be so bad to have another dude chiming in."
Pride aside lets get real, sex gets boring 1v1 after some time, because girls all look and feel the same, and there only so many roleplays you can do. That's why I sometimes hate hentai on how they portray sex as this never ending bliss, because truly you get bored afterwhile, your back hurts, prostate pains, and sometimes get these "friction burns" on your thighs.
This comment sounds grander than the reality is, so don't be fooled. I'm just the average guy, who happened to have groped a few girls with consent at drinking parties and had 1 girlfriend in his lifetime with a real libido.