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So, if put on the spot I'd say that Cullen was physically a virgin, but had very vivid memories of a sexual nature.
I agree with this interpretation, it was aptly put. Cullen's devotion to the Chantry throughout the first two games also influenced my thoughts in this regard, because iirc Andraste was betrayed by her husband and that's the reason the Southern Chantry has solely female Mothers. Although the Chantry does not ban physical temptations outright, it is clear they are sometimes very strict about it (based on what Alistair said) and that the female is relatively highly regarded in matters of faith at the least. Cullen also took Chantry teachings pretty seriously in the beginning, far more so than Alistair. That's one of the reasons I feel like Cullen wouldn't personally want to just use a woman that way without some level of romance involved. There's also the fact that, as someone mentioned earlier, when the Inquisitor first questions him about whether physical temptations are banned Cullen immediately jumps to saying that marriage is allowed (but that some people may choose to give up more to prove they're devoted to he Maker). I think it speaks loads about his preferences that he'd think of commitment as soon as sex was mentioned. But, again, that's my interpretation.
Alistair and Cullen are joint first for me. I don't think I could pick a favourite out of the two because both a good in different ways, despite both being kinda awkward and completely infatuated with the corresponding protagonist.
Same, I loved Alistair and Cullen in different ways for different things based on their personalities. I know a lot of people like to compare them because they are both ex-Templars, but it's very refreshing to see that they both are quite dissimilar as well.
First post here! I let my Trevelyan flirt with just about anyone at first. I thought she'd go for Iron Bull but his nipples distract me so, and then his comments about *cough* makeshift toys and threesomes just WHOA. Blackwall just seemed way too old for her. I haven't gotten my first PT yet...I just think something's really up with him. But once she started flirting with Cullen and made that dorky smirk in Haven, I was hooked!
I wonder how an age dynamic would work between them. My Trevelyan looks like she's barely out of college and getting past her "ugly duckling" phase in a way. I mean she acts like it with her headstrong questions towards Cullen. It reminded me of a kid asking too many questions at first. Cullen's questions on her well-being made me think he'd take her on an imouto kind of path.
I'd imagine Cullen's got at least 8 years on her. I took his hesitation more like, "you're a little young and I don't want to look like I'm corrupting you". What reasons do you think Cullen fell for their Inquisitors?
I think Trevelyan's actions more than proved her maturity in that regard. Cullen himself is very, very capable when it comes to matters of work (he was made Knight Captain around his early 20s, for instance) even though he's more socially awkward at times, so he's probably aware of the hardships associated with being younger in a position of great power. Especially if he had to deal with any flack he got for being in charge of people older than him in DAII. Seeing Trevelyan prepare to risk her life for the good of the people really made an impression as well. Some people can be remarkably grounded at a young age, so it doesn't need to be an issue if you don't imagine it to be 
Dumb question that's probably been asked too many times, but are Valuables in your inventory stuff you can safely sell?
You'll want to make sure you've given in all creature research materials beforehand since they are kept in the same folder, but after that, yes.