there's a common conception i've seen in fanon that between simon and isabelle, it's the latter who is the top/dom. i happen to hold the opposite belief (currently and to an extent). this is an explorative "essay" based on the thesis that in any roleplay/bdsm/etc. that takes place in their bedroom, simon is the primary top/dom.
there are a few significant threads in simon and isabelle's romantic arc and relationship that back up this thesis, the first of which i've actually discussed in this post regarding, primarily, trust.
isabelle learning to trust simon with her heart is, in many ways, her learning to entrust all of herself to him; her vulnerabilities, her passions, her deepest cuts and greatest loves. by the end of the series, she's managed to lay herself bare before him in a way she never has before, and her conversation with jordan about not being able to trust anyone who isn't related to her, followed by this line to simon and the resultant biting/feeding, is a clear communication of this.
before simon, isabelle was likely almost always the top. i don't know how much kinky fun she got into before him, but at the very least, i do believe she was always the one in control. because that was what she needed. holding intimacy at a distance was the only way she knew to navigate, well, intimacy, without it biting back. so when she did allow herself to dive into true intimacy with simon and all the trust and vulnerability that required, it makes sense that her stripped walls would leave her on her back, willfully offering up control as an act of love, an expression of trust, and the desire to sink into the security she's come to know in his arms alone.
of course, this isn't to say she completely loses control as the bottom/sub. simon respects her too much not to listen to her and abide by her wishes. the lord montgomery scene is a perfect example of this dynamic as isabelle purposefully establishes a scenario where she is in the role with less power.
simon's response to isabelle's initiation is the result of an intersection of this thread of trust and another that follows simon claiming his identity and worth. if we back up to the biting scene, we see that simon doesn't believe he's a worthy candidate for isabelle's trust---not that she can't trust him, but that she's been so guarded and he doubts whether he's worth letting those walls down for. it's his previous actions that make him hesitate, this lack of certainty he's developed in himself, in his desires and abilities. as soon as isabelle convinces him he needs to stop trying to hide his true self from her, his instincts take over and he's on top of her, administering pain and pleasure.
the immediate aftermath of that moment is full of questions and communication, a check-in on isabelle's wellbeing, a confirmation that they both enjoyed it.
this enjoyment was, of course, partially the nature of vampires and the process of blood-sucking, but in the context, there's an undeniable connection to the aspect that was handing simon the reigns and trusting his actions and instincts. isabelle is as much telling him that she liked him taking control of her, liked seeing him take what he wanted without denial or embarrassment or humility.
this communication and understanding, but also the confidence that simon gained from the conversations and experience, allow him to respond to isabelle's lead in kind when she prompts him with the concept of lord montgomery.
here, they are comfortable to further pursue the roles previously established---isabelle in her teasing manner and simon with an amount of surety that, yes, takes a second for him to grasp, but that he eventually claims.
early in the series, clary tells us that simon is the kind of person who knows what he wants. as the events of tmi occur, we see this aspect of his personality challenged repeatedly as he's pulled in a number of directions after having lost all that was stable in his life, all that he knew he'd wanted from it. it's this simon we become familiar with through the books, but it doesn't negate the piece of him that likes to function in that definitive way. we see it in his argumentativeness, in his ability to come up with a plan and stick to it, and, finally, in the end, we see it with isabelle. when his spinning compass finally settles, it lands facing her, as the only thing he really knows he wants. and, no, that determinedness is not necessarily an indicator of a more powerful position in the bedroom, but in the context, we see it line up with something like a reclamation of control. there's a space in which he knows what he wants and how to get it, and that space is with isabelle. when so much of his life has fallen out of his grasp, there's something to be said about the peace and thrill he might find in being able to exert some amount of control.
on the other hand, as previously stated, it's the relinquishing of control that isabelle finds herself practicing as she's pursuing simon. she's been careful with her heart for a long time but at her core isabelle wants to be free and reckless, and with simon she finds a safe space to open herself up to that lack of dominion.
all this to say, it does make sense why the standard understanding of their sexual dynamic is entirely opposite from the one i've proposed here. isabelle is more experienced; simon is more tentative with physical initiation; isabelle is the type to charge while simon likes to assess first.. but the counterintuitive nature is precisely why i think the reversed roles work. they're each other's exceptions.
