I just knew it was wrong
Yep.
It's wrong because you're describing pressuring someone to do some kind of sexual behaviour. It doesn't matter whether that's with you or with someone else, it's still sexual pressure. It doesn't matter whether you had some earnest wish or intent for that sexual behaviour or you intended it as a joke, it's still sexual pressure. The word for sexual pressure is coercion.
I know that's pretty blunt, but that's where we are with this, really.
Having it be a joke doesn't really help or make it lighter in this situation. For a start, you're familiar with "it's a joke only if everyone's having fun"? Your boyfriend and Jill's boyfriend - and right there, can we have some different names for these people? They're owed more respect while we're on this topic, especially when you and Jill have more serious names - weren't having fun, so it was no longer a joke.
To illustrate how joking doesn't make pressure lighter, let's step into the shoes of a person in your boyfriend's position for a moment.
When we like someone, want to please them, want them to like us - and those are usually pretty strong about a boyfriend or girlfriend - it often feels difficult to say no to them. It feels uncomfortable and wrong, and we get an inner conflict because we want to say yes to them, we just don't want to do that thing they asked. So now we're battling with ourself as well as with our partner. And they're joking: that must mean it's no big deal, so that must mean there's something wrong with us for feeling so strongly that we don't want to do the thing. We want to say "No; and finally, no. Stop keeping asking: uncool, and knock it off" but they're
joking, so what we think and fear we would actually be saying, if we said those words, is "I have no sense of humour and I am a giant wet blanket and boring and overly harsh and I am being Dramatic and Hugely Over-Reacting and am the least chill and least agreeable person ever". And we are confused that somehow we feel so bad and confused with this person we like so much. And we don't know what to do, and sometimes we're not able to think clearly about what to do any more. We just want it to be over.
That's how that kind of pressure and coercion function. You see how, if that person does the thing, they were
not consenting, because they weren't able to freely choose whether or not they did it?
Why would you want people to kiss each other when those people don't want to?
I do also want to acknowledge the gender dynamics here and say that because of (bullshit) cultural messaging about guys being more in control, guys can have a particularly difficult time if they're on the receiving end of sexual pressuring, because they may not have known or understood that that's even a thing that can happen to them, and they may think that it shouldn't be any trouble for them, and other people may not see it as a big deal. No matter anyone's gender, sexual pressuring is rough and hard to deal with.
(Just a note: I recognise there may be some celebrating going on, so I'm asking that you reply here only when you know you're in the right headspace and capability to do so, so we treat this topic in the way it deserves. Too, I see what you opened this post with, and I'm genuinely very sorry that I'm not able to celebrate with you as this stands in the here-and-now; you're a valued member of our community, and I want very much that we can work through this part so I can get (belated!) party streamers out for you elsewhere.)