forced to watch
When a character can only watch a sexual intercourse, being unable to either partake in it or stop it from happenning, often causing sexual frustration. Said character may be bound, restrained, forcibly held back, be on the other side of a see through obstacle they can't get through (ie: a gate, a pane of glass) or otherwise prevented from acting upon the intercourse. Often associated to cuckolding and/or rape.
This tag implicates forced, looking_at_another (learn more).