Love is, I'm sure we're all aware, a many splintered thing and a four letter word. Regardless, it is a very powerful motivational tool and plot hook, and it has created some of the most memorable moments in my games.
In my old campaign, which is sort of considered by our group as our "glory days", by the end of the campaign pretty much everybody was paired off, and it happened with tact, subtlety and, most importantly, entirely organically without any spurring on. The wizard, who was in additional to being a wizard a manipulative, ladder-climbing, Vetinari-like plotter, had married his nation's ambassador, their romance being underplayed to comic lengths. I had established that the upper classes were very much like Edwardians, and any display of actual affection between the two would have made them "common". Regardless, the role-playing rapport built up between PC and NPC made it very clear there was something there, and the "wedding" episode was hilarious as the other PCs devised all kinds of 'wedding presents'; retrieving some of them were side-quests in their own right.
The party's bard finally gave up promiscuity and was enamoured with the 17 year-old daughter of a villain who they'd 'resuced' from his clutches. She clung to him like glue, and became the "work experience kid". In the final adventure, the NPC girl's role was taken by the bard's player's real girlfriend, which created exactly the right chemistry and led to the canon assumption the two wed (the age difference is not substantial by historical standards).
One of my personal favourite moments was the introduction of an arranged marriage for the dwarf fighter. Way back at the start of the campaign, the player and I had established the dwarf was exiled, but we never got round to the 'why'. The PC told everyone it was for desertion, and then let on it was for trying to poison his chief, but the actual reason, when he finally owned up, was to escape an arranged marriage. Eventually he went home, regained his honour and position, and returned to the campaign after a sabattical - at which point his fiancee showed up! I played her as an honest, tell-it-like-it-is type who was very smart and quite feminist for a dwarf (dwarves in that campaign were highly patriarchal). The two of them decided..what the hell. Let's try and make this work. And it did. By the campaign's end they'd learned to like and respect one another, and there was the faintest hint that this might go further.
The only PC who didn't end up "happily ever after" was the druid. I suggested he could try to start a relationship with the talking gorilla, but my suggestions were politely rebuffed.
We have fond memories of that campaign, and those NPCs, particularly the ambassador and the girl, rank extremely highly on everybody's list of favourite characters.
In the campaign that followed, romance was pretty much left out, with two notable exceptions. The sea elf ranger (who was actually a sahuagin but this doesn't matter here) became, entirely at the player's instigation (and somewhat inexplicably and arbitrarily) smitten with the rigid, stick-up-the-armour paladin. She (both players were male) flirted openly with him and he, hilariously, always found excuses not to be there. It was classic role-playing and very funny; then the PC left, and the sea elf transferred her affections to someone else - and it all went round again.
In the same campaign, one PC was a woman disguised as a man. Unfortunately, she shared a bunk with the ship's resident Casanova. Her disguise having failed to fool him (he could smell a woman at 500 yards), the two established something approaching a friendship. Later, the man in question havign got into trouble with the law, he abandoned the PC and fled - I was always planning to establish that this was because he was in love with her and didn't want her to come to harm, but I never got around to it before the campaign folded.
I had a campaign where the young prince and heir to the throne formed a strong bond with the street girl who got caught up in events. While the other PCs plotted his restoration, she taught him to juggle and grift people, and how to lie. She was a corrupting influence; had the campaign gone further I'm pretty sure something would have happened there.
In short, love and romance in D&D can be a great role-playing adventure and the source of very fond memories. Don's experience sounds unfortunate but it sounds like a case of sour grapes on behalf of the player. It probably works best if you skirt around the issue of sex, or at least don't feature it prominently. There's usually one lecher in any group of characters in a STARP campaign (with telling frequency this is me), but it's generally sidelined. Nobody minds a bit of romantic flavour, but people tend to get a bit awkward when you bring sex in. I did once impregnate a player character (stop smirking! You know what I mean!), and had my old campaign continued I'm sure the patter of tiny feet would have been heard eventually, but in general, it helps if the campaign has nothing more than what you'd get in a PG-rated romantic comedy - "Contains sexual references" might be as far as you can go.