This is an odd topic I've come across in a few books I've been reading this month. I don't feel that when reading a book I should have to wonder whether or not two characters just had sex, and yet recently I've found myself having to re-read passages more than once to clarify the question. Some things definitively happen in fiction: no one kinda dies, or kinda gave birth. Sex is much the same.
This kind of nebulous writing appeared in a brief passage in Sweethearts, and Everybody has Everything; both of which I loved. Both of those authors are extremely competent and capable of communicating extreme detail with minimal words should they choose to do so; and so I feel the vague 'Did they just get it on?' atmosphere they created in their respective books must have been intentional. Only I don't think there is any benefit to casting this cloud of obscurity.
In Sweethearts, I tried--and failed--to excuse the passage by telling myself it's YA fiction and sex isn't the focus. I couldn't let that pass as the hook-ups in that book were more than likely psychologically damaging and the act happened for wrong reasons. In the case of Everybody has Everything considering the concrete realities that the book was depicting from start to finish for the life of me I don't know why 'oral sex' couldn't be communicated in some more clear fashion.
Authors can be artistic and vague when talking about sex, they can blunt; but reader confusion is never a good thing; especially so since sex really ain't that subtle. I wish I had marked those passages so I could quote them here but this complaint brings to mind a grip from a review I did awhile ago for Breakable You.
" "They had spent the afternoon on the couch, though, come to think of it, he couldn't quite remember whether they'd actually, technically, made love. They'd done something, but he couldn't quite remember what." I don't even know what that means. Perhaps if we replace 'made love' with 'bad acid trip' I could make sense of this, but when people aren't drugged they definitively know whether or not they 'technically' made love or not. No matter how pleasant having sex is, it isn't that subtle. "
That's how I felt then, and concerning blurry sex that is still how I feel now. I don't need exacting detail or erotica. Hell, the page break in Gone with Wind is one of the more profound sex scenes in fiction that never actually happens on the page but it is communicated with zero confusion. If it is important to your story and character background, make that information clear.
As The Allman Brothers sing, 'Please don't keep me Wonderin.'