They said it glowed when they touched it so I assume he didn't need to have sex. Sex is probably more recreational for the main couple despite saying it was for purification. If it was only for that reason then he wouldn't have so strongly denied creating an heir.
He never had sex with his lover's dad. He just touched him. The lover had sex with him because he obviously wanted to have sex.
kissing or sex in human world makes purification easier, because of the direct exchange of chi in a contaminated world. I think because they returned, there is no real need to direct exchange with the dragon king, the enviroment is contained and pure enough that simple touching already releases a strong force of purification. but since it is a ability that consumes strength and the dragon king is considerably tainted, it consumes a lot of life force, hence the sacrifice. that's what I thought.
So....he shag his lover's dad to purify the dad's sphere? Wow...talk about sacrifice.