I've been married for 7 years (my husband and I met playing online.. unsurprisingly). I've RPed some rather long term romantic relationships with several people, but I've had the good fortune of having very meaningful, enriching experiences. Looking back I belive it had to do with setting clear boundaries as to what I was/was not comfortable with. I think it sends a clear message straight away and avoids any possible drama that might arise when under pressure. I don't go looking for romantic relationships and I never play overly flirtatious characters (I would be pants at it anyway), so if an opportunity for a relationship IC starts to blossom, I tend to be very cautious (or oblivious, depending on how you look at it). It will almost always be a very long journey.
My OH has no issue with me RPing relationships... I once played out a rather violent shouting scene with a good friend whose character was deluded into thinking my character was strongly attracted to him. Nothing physically/mentally abusive happened because we set down boundaries early on. Perhaps this directly influenced the outcome but in my opinion no one should feel they have to act out something they are uncomfortable with for the sake of staying IC. I believe it is most important for the person mentally and physically behind the toon to stay safe in all respects.
I've seen too many friends hurt by bad, abusive, or blatantly manipulative relationships IC and OOC and I would shudder to see it happen again.
Not to go all depressive mode... IC romance is great when all goes well. Nothing wrong with a good old snogging sesh! I just like to play the (pretend) voice of Reason sometimes.
~ Retired, but thinking of brushing off the dusty tome ~