In Dragon Age: Origins, there are four romanceable party members. In two cases, Morrigan and Zevran, you can have sex quite early in the relationship, as both are quite casual about intimacy, but only later does attachment really begin and an element of love possibly grow. It begins with sex but doesn't end there - or, at least, it doesn't have to.
Another example would be what happened with my first playthrough of the game: the romantic relationship between Alistair and my Warden did not culminate with sex, which happened for the first time maybe 70% of the way through the game, but in the final hours of the game. After being made king (a role for which she'd supported him), even though they were both in love, he ended their relationship because he felt it was incompatible with his duties as king. At the very end, though, in the final battle, he sacrificed his life to save hers, stealing the death she had planned for herself, because he still loved her. For Wardens who don't have this romantic relationship, he can be talked out of making this sacrifice, but in cases where he's in the love with the Warden he doesn't give a choice, essentially deciding the end of the game for you. He does this regardless of whether you've slept together or not. However, if you do share sex, it only highlights the intimacy of the bond that your characters eventually lose. The culmination of this romantic plot was not sex but his noble death and her deep bereavement. Even after winning the game/battle, she felt like she'd lost everything.
In another case, to get back to Morrigan, a sex scene towards the end of the game that involves her is about far more than titillation. Depending on how one views her character, it can be about secrecy, freedom, betrayal, dubious consent, or a whole host of other themes. She wants to have sex with someone, whether the male protagonist or one or two possible party members who are Wardens, in order to conceive a child that will have the ancient soul of the corrupted creature you're about to fight. She claims that the child will be free of what taints this enemy. You have pretty much no way of knowing if she's telling the truth. And if you're a female Warden and want to take her offer, you have to ask someone else to do it. In the case of my female city elven Warden, that was Alistair: someone she loved even though he'd just ended their relationship, with whom she could never have a child (one of the reasons he ended it), and someone, very importantly, who strongly dislikes Morrigan and is not interested in her sexually (hence the dubious consent). For my Warden, deciding whether to persuade him to do this or whether to tell Morrigan to take a hike was absolutely nothing to do with wanting to see a sexy scene... In fact, this was one scene I did not want to see, which would have made my skin crawl. And turning it down was what caused my story to end in tragedy.
How the frack is this at the level of beach volleyball?
My memories of Dragon Age 2 are not as clear, since I've only played it once. Even so, as far as I recall, sex scenes occur in the second act of three, with romantic plotlines continuing through the third act. In one example, a character is scared by the feeling of getting "too close" to the protagonist after sex and the romance only resumes much later, after character development. In another example, a particular romantic interest doesn't want to have sex no matter how "nice" you are to them throughout the game (another of your criticisms), but the romance can still continue chastely. In general, various characters react to the evolving relationship as the story goes along. Some of the actions you take with regard to your protagonist's romantic partners can change the tone of the ending, and whether or not one wants to punish a guilty terrorist in the game's final hours may depend on the romantic bonds that have built up.
Even in the case of DA2, which, during my playthrough at least, showed poorer writing than DA:O on the romance front, it still isn't true that sex scenes are the payoff and the end of plots, that they are merely tacked on. It's also not true that sex is something characters "have to" have just because you're nice to them. There are dialogue options that can end relationships permanently, because a character will not be interested in a protagonist if they do certain things. There are also characters with whom flirting is possible but romance will never happen, e.g. Varric and Avelline; their friendships are genuine, not some despised friendzone.