I do not base this on any surveys or empirical data, so this is purely speculative, but I'm assuming that in many cases most pen and paper gaming groups are male so romance would uncomfortably force homoeroticism onto the group.
Is it a problem that rpgs don't contain much romance?
No. I don't mean to judge groups that do have some form of romance in their game successfully (and if you do, please share how you go about it) but lets go over how romance in games could work out:
1. Two same sex players can have characters in love (either both male, both female, or male and female).
2. Two opposite sex players can have characters in love.
3. The GM can have an NPC in love with a player's character (male or female).
One could add further options with polyamorous groups of lovers and the transgendered, but I'll stick with the above three. In 1. I've already mentioned that homoeroticism in gaming could just creep some players out - even if they're fine with homosexuals that doesn't mean they want to pretend to be in a gay relationship. In 2. this could weird one of the players out, or even if it didn't it might make the rest of the gaming group feel awkward - though this isn't necessarily the case. In the third option this meets the same problems as 1 and 2.
I will say that I do think it's odd though that we very easily will kill vast numbers of fictional beings in games, but acting out something meaningful like a relationship or pleasurable like sex intuitively feels very weird and inappropriate. I take these intuitions are common. Am I wrong?
However, I will note that this is different than in mmorpgs. I've attended weddings of characters on rp servers for WoW. This seems to be easier online, and I assume it's because we're not connected to other people at the personal level we are with players around a gaming table. Any other thoughts on this?
Books on this topic:
Blue Rose offers romance in the more general sense that is often found within the fantasy genre. The game focuses less on combat and more on role play and interacting with others. The kingdom within the game is very sexually open of gay/lesbian/transgender lifestyles as well as causes like environmentalism.
Book of Erotic Fantasy offers more of a practical guide on how erotic romance can be put into games. The book also covers everything from playing a character who's pregnant to the often overly sexual nature of actual polytheistic belief systems and the absence of this sexual element in the polytheistic systems in most games.
Breaking the Ice: Actually looks like a really fun date rpg. In that context, a romance game would be appropriate and probably a great way to get to know one another.