Transcription:
A 4chan post by Anonymous with a picture of a young girl with pointed ears cradling a large sword in its sheath
>Group is having a party for cleaning a goblin cave that's been annoying the village
>Saved the lord's daughter
>BBEG shows up
>Says he has come to claim his virgin wife so that he may perform a ritual to summon a demon
>Obviously refuse to hand girl over
>Fight breaks out
>Bard grabs the girl and runs away
>Seduces the girl whilst the rest of the group try to fight the BBEG and are failing
>Has sex with the girl
>No longer a virgin
>DM is furious
>Angry that we completely destroyed this plot
Does the DM have the right to be angry that we poked a hole in the plot to beat the BBEG
The DM allowed the NPC to be seduced. It's been said a million times but clearly that DM hasn't figured it out yet, the Persuasion skill is not mind control. Tell the bard "she just isn't interested and your continued attempts to woo her are just making the poor woman despise you more and more every time. In fact, you think you might have heard her muttering something under her breath about taking her chances with the big guy in the spiky black armor. Roll perception. She appears to have slipped away while you weren't looking."
If the bard decided to force himself on the girl that would open a whole can of worms but even if the table is okay with such behavior you've now pissed off the girl, the local lord, and the BBEG who (as others have noted) could probably find another suitable subject for his ritual with a modicum of effort. After he finishes beating the crap out of the bard.