Implementing the romance can hurt the end user, if much of the character content of the NPC is locked behind the romance (note this doesn't hurt just the romance hater, but it effects all users presuming multiple romances
Some can - and have - made the argument that a proper romance is beyond the scope of current games. However your argument here is a strawman, because you're abstracting a specific point of gameplay to the larger idea of the game itself. One only need to look at any game with a maligned minigame (which right now, Romances are treated as) and you can understand the idea. If you can't implement your lockpicking idea without it being frustrating to the end user, why implement it?
They keep repeating it because
Developers have made this point.
people who aren't developers keep saying this point is invalid.
I also find it charming that you think "writing a romance" as not being difficult, when its as difficult (or not) as any other thing you write.
But as for resources, lets look at a theoretical implementation of a romance choice.
Romance Option (RO) needs to be a fully fledged character whether you romance them or not
This means RO needs to have two dialogue trees, one for the romance, one for not having the romance
For RO to be a character (and not a puppet of the player), there needs to be conditions for a started romance to fail other than the player failing it. This means that some way of tracking this must be created and implemented
Any reaction that RO has to story elements needs to have two reactions, one considering a non-romance option and one considering a reaction consistant with the progress of the romance
The world itself needs to recognize the romance; what does the rest of the party think of the romance, if you stay at an inn, do you need a different rate because of a private 2 person room, do people recognize you as a couple.
Should there be romance specific quests, what happens with these quests if the character doesn't romance RO?
Does the scope of the game make a romance make sense? If you meet RO today, and the game only covers a week of life at best, is there enough time to develop a full romance?
Should RO get mad if they are left out of the party? If you don't talk to them? Are they jealous of other ROs?
Does the PC get affected by what happens to RO in combat and vice versa?
Are there any systems that need to accommodate the romance?
Probably much more to think through, but I hope this points out a romance is not a trivial thing tossed into a game at a whim with no cost to other parts of development.
I don't believe anyone has been advocating that Romance = Sex. I have seen many people dismayed, in fact, with the lack of romance options that don't give a gratuitous sex scene before the climactic last fight, bioware style.