I have these sims that I've been denying romantic relationships to for a while, because I wasn't sure who should get together with whom. Eventually figured it out, and yesterday I let two of them get together.
So, it's pretty obvious that the poor things were desperate. They were already Best Friends, with maxed-out relationship. (100 in both Short Term and Long Term relationship). This means it took one romantic interaction for them to fall instantly in love with each other. Just a Flirt --> Caress, and they were In Love. Not a Crush, straight to In Love. (The pink hearts floated above their heads followed immediately by the red hearts followed immediately by the big red heart with an arrow through it.) Then, the girl's Wants changed to Talk to Boy, Dance With Boy and Get Engaged to Boy, while the boy's changed to Entertain Girl, Dance With Girl and Get Engaged to Girl.
Flirt --> Caress is a light touch on the side of the face - stroking the other person's cheek. They hadn't even kissed at this point, and they wanted to Get Engaged! Okaaaaay. Y'know, even in my wildest teenage girliehood, I don't think I ever wanted to get engaged to someone I hadn't even kissed! I don't even think I wanted to get engaged to the first guy who ever kissed me "properly" (who was, btw, the bass player in my favourite band at the time - true story!). Poor little sims. I've clearly been neglecting their emotional needs.
Then the boy took the girl out on a date (as I was in his house at the time), and I ignored the Want to Get Engaged. And some more sensible Wants spun up. They wanted to Dine Out together, then they wanted to Hug each other, then Kiss and Make Out. (They made out in the middle of the road, with cars hooting at them - something that would be rather dangerous in real life.) After that, both of them had 3 Want slots clogged up with Have First Ever Woohoo With (the other), Woohoo With (the other) and Woohoo In Bed with (the other). So I relented and had him ask her home. She autonomously ran up the stairs to the bedroom, in front of him (!), and they had fantastic sex as only sims can (lots of giggling under the blankets, strange acrobatics with random hands and feet coming out of the bed, and fireworks going off overhead). When they got up again, the Get Engaged Want was back.
Who am I to ignore what is clearly Meant To Be? :D
Update: They've now been on four dates, all Dream Dates, and as a result are both walking around with permanent Get Engaged and Get Married wants. I didn't even realise Get Married could spin up as a Want for sims who aren't engaged yet!