Note: Just a forewarning: not smut, only Rizzles if you squint. Look elsewhere if either of those is what you seek.
There were multiple reasons why Maura Isles liked sex.
As a medical doctor, she was well aware of the various health benefits—stress relief and the release of immunoglobulin A, among others. It was a perfectly healthy activity, and such archaic ideas as "virginity" or "heteronormativity" had only been mentioned scathingly in her parents' household ("though of course a young lady should think about whom she sleeps with, perhaps, still" hovered in the back of her mind).
But those were the less personal reasons, the ones she felt comfortable speaking of to the world at large—because while Maura Isles, M.D., might not always have a filter for what were or were not socially acceptable conversation topics, she did have a filter for what personal things she told whom.
The first time she was intimate with anyone, she surprised both herself and her roommate—her partner, Céline, during her last year at the boarding school—with the tears that leaked down her cheeks. They were happy ones, but a slightly drunken Céline had held her awkwardly, repeating "Il te faut pas m'en remercier, arrête, s'il te plaît, Maura, c'est bizarre" in response to Maura's repeated, slightly slurred whispers against the French girl's shoulder. Things were awkward between them for the rest of the (thankfully short, by then) school year.
It felt wrong, somehow, not to thank someone for giving her that feeling of intimacy, of belonging, of being appreciated and wanted for once in her life. But after Brandon, her first boyfriend in college, just stared at her when she beamed at him with moist eyes, she learned that it wasn't really welcome.
So she set out to thank them in other ways, and it turned out that once word got around how good "that quiet little genius girl" was in bed, she could have her choice of partners.
Ironically, that worked against her, as they came for the mechanics and often left her little in the way of a connection. Still, she took what she could, and endeavored to persuade them to stay around for as long as they wanted to—unless they stopped one of her rambling tangents by attempting to drag her forcefully to bed, and then she threw them out unceremoniously.
And then it wasn't quite so easy any longer, once guys passed the "horny college age" (girls were generally more discerning then, too, but then they were more discerning, period, generally), and she wasn't sure whether she was relieved they might want her for her now or disappointed that they still didn't, and she missed the imitation.
Maura never knew why Jane Rizzoli did seem to want her around, but she was grateful enough not to question it. And once she though Jane might actually stay around—maybe around the one-year mark (generally people were tired of her by then, or turned into something a little more serious, like Garrett or Ian)—she threw herself into the relationship whole-heartedly.
Because Maura Isles knew people liked you for what you could give them, and she loved to give—anything she had, for the smallest smidgen of affection in return.
Jane Rizzoli was generous with affection, if she thought you needed it.
Maura wondered how Jane could see it, when no one else seemed to. She supposed it was one of those "gumshoe" things. Maybe one special to Jane—but then Jane was one of Boston's best detectives.
She wondered if Jane wanted her to return her affection in bed, too: the way Dr. Maura Isles knew how to all-too-well, her way to show her gratitude for any kind of emotional connection, the way to reciprocate that felt natural to her by now, like breathing. But she didn't know.
And Maura never guessed, not if it was important, especially not when a wrong guess would mean losing one of the best things that had happened to her.
So she flirted, and tested the waters, and was never really sure—because Jane was Jane, and she was affectionate and gentle and never hesitant, and they had always been that way, hugging and touching and comforting.
Maura Isles wasn't sure if she was relieved—grateful—or disappointed when Jane Rizzoli apparently didn't want to sleep with her.
Author's Note: French translation: "You don't have to thank me for that, stop, please, Maura, it's weird." (My apologies for not including this sooner.)
So this is maybe kind of sad. But I have to say that this is my interpretation of canon Maura, some of the consequences of growing up as an attention-starved little girl. I'd love to hear if you think I'm on the mark or not.
(And even in canon, let's face it, she'd be totally up for doing Jane if Jane wanted.)
And it was the detail of Maura getting very emotional during sex from AnExhibition's "The Commitment" that (loosely) inspired this fic at all, so... yus. Credit must be given where it is due (and it goes without saying that the amazing Maura Isles and the badass Jane Rizzoli and all the other R&I characters are not mine).