There has apparently been a lot of discussion on tags that can clearly communicate the difference between rape and forms of consensual use of force. One solution that has come up is the forced_partners tag. It's been mentioned here and there in the forums, but as of yet, it hasn't had its own post.
The tag is what it sounds like. For those unfamiliar with it, it's where a third party is forcing other characters to have sex, and at least one of them is unwilling, at least somewhat. The character may simply be uncomfortable or embarrassed but still consent. There are situations where that would overlap with rape, but it's mostly a BDSM sort of thing.
For example, a mistress forcing two straight slaves to have gay sex would fall in the forced_partners category. Even though they don't want it, they have consented to the slave role and whatever may come of that. It also applies to images where the third party in question is not visible, but is clearly responsible.
There are just under 100 images tagged with forced_partners at this point, but it's steadily gained traction since its creation. There are probably a lot of applicable images that come to one's mind when reading this post. I'm writing this to raise awareness that this tag exists for this particular situation, which for many is a fetish. I would encourage users to tag applicable images with the forced_partners tag.
Updated by Furrin Gok