I seem to be having the exact opposite to most here it seems, before the patch I had a lot of trouble getting the eternally faithful moodlet, the only solution I found was to not let them live together, have them just date and be romantic in public for ages then only move in after getting the moodlet or it would never happen once they lived together. Sometimes it took weeks to show up.
Since the patch it appears at exactly 20 days every time, living together or not, I still have them be romantic in public but I found during the baby boomer they got it and the only people who really saw them be romantic was all their kids. And this was while getting a truck load of gifts and letters from other people every day. I think my max mail was 75 in a day, crazy.
As for it vanishing, yes it does do that when you travel, but i've also had it come back when I got home or even had them get faithful while on vacation then it goes back to eternally faithful after they return.
I like it not just because it's a nice moodlet but also because my sims spend an awful lot of time thinking about it, like it means a lot to them to have it, I find that rather sweet.