Anyone who's having sex already - Too busy to stop you - Lust/Bonding

Anyone who's having sex already:

You can join in on any sexual or just lustful act that you stumble upon. Neither partner will stop you, but they will be aware of what you're doing and it might surprise them.

Too busy to stop you:
Whoever you join in on will be so hot and bothered by the act they're engrossed in that they won't think to or even want to stop you from joining in on the fun.

Lust/Bonding:

The more submissive people involved will just accept the added stimulation, while the more dominant parties will see your actions as an act of assistance. They won't likely say "Hey! You fucked my partner!", they'll more likely say "We just fucked the hell out of my partner!"