(Here we go again...)
Whether Solas slept with Lavellan or not is entirely up to you. Personally for me, when I heard Solas say those lines in Trespasser, I thought it was confirmation that they did sleep together. Otherwise, why bring up the subject of sex at all?
Others interpreted it as he didn't. Both are valid.
Again, I'm gonna quote someone on Tumblr for the explanation:
http://ar-lath-ma-vh...at-the-line-was
"I see two interpretations of this exchange.
1) Lavellan is referring to sex. She’s saying that he took advantage of her, the implication being that he simply used her either for his own pleasure or in order to manipulate her - or both. He denies this, claiming that he never would have slept with her simply for the sake of his deception. In other words, his feelings for her and their relationship were both genuine. His desire for her, sexual and otherwise, was not a tool for controlling her or falsely earning her trust.
2) Lavellan is using the phrase strictly in the way the Dalish do. He deceived her and toyed with her. Everything about their relationship was a lie in order to use her as a pawn in his plot. He tricked her into thinking that he loved her and took advantage of her trust. He denies this and reinforces it by explaining that he would not cross that line of intimacy with her when she still did not know the truth.
Headcanon away. It’s meant to be ambiguous."
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And of course there is Patrick Weekes' tweet.
And another:
"if you want to say solas and lavellan didn’t have sex that’s totally your right because weekes said he left it up to player interpretation.
which means people who think they did have sex are just as right as people who think they didn’t.
Solas does not say he never slept with lavellan, he says he would not have slept with her under false pretenses, which really just means she wasn’t a conquest for the big bad dread wolf, his feelings for her were 100% real and legitimate. you all just read the ‘i did not lay with you’ part and that’s it.
again, it’s a very valid interpretation to say they didn’t have sex, but it’s also just as valid to say they did."