Lover’s collars are another matter entirely, very different from the protection/working relationship kind of collar that Jasper had to wear! Lover’s collars are the most voluntary kinds of collars there are, and a symbol of genuine emotional connection and willingness to devote oneself to their lover!
Jasper (Underfell Sans) really wants to see his s/o in his collar at some point. He won’t push for it real hard, because of how intimately he understands the negative side of being made to wear a collar you don’t want to, but the mental image of his long-time partner, wearing a collar he gave them and that they graciously accepted... It kinda gives him the warm fuzzies right down in his soul and makes him feel real lovey towards them~
But if they’re not comfortable with it or just don’t really wanna, hey, that’s no skin off his nose-- he doesn’t even have a nose, haha, amirite? Nah, though, there’s lots of other ways to express a romantic connection, human customs but hey, when in Rome, y’know? He’ll get ‘em a wedding ring or some other kind of trinket like that to advertise their relationship, something they can both be happy with!
Pyre (Underfell Papyrus) on the other hand... he’s a little more reluctant. He likes the idea of the collar and the connection/marking it represents, but actually going through with it... Well, the last time he gave somebody a collar--just a platonic one--it ended up creating a pretty huge emotional rift between him and his only family in the world and that’s enough to make anybody hesitate, especially when it’s his relationship with his lover on the line this time!
He’ll probably get a collar for his partner long before ever saying a word about it to them and when he finally brings it up...or more likely, when they finally stumble across it hidden somewhere and ask about it...if it’s something they agree to do, he can absolutely not be the one to put it on them. It has to be entirely their decision or else he won’t feel right about it and will always kinda doubt if it was something they actually wanted or if he pushed them into accepting it.
...But if they do wear it, it’s absolutely the hottest thing for him and he gets very passionate about it whenever he sees them wearing it. They chose to wear his collar, they want to be his and he is so deeply, madly in love with them he can hardly stand it!!!
Hey, he’s a complicated dude: he’s allowed to have mixed feelings, even wildly mixed ones from such opposite ends of the spectrum! XD