You shouldn't be getting to the point of making vows without being perfectly clear what you need from a partner and what other needs you have. If you want a side piece or side pieces your main LTR should know that and be ok with it.
The second she says that, she's externalizing that she's thinking about having sex and doesn't want to be seen as a slut. This is the point where I excuse myself and pop a viagra, but you're young so not necessary. Ignore and continue escalating.