this is a good possibility

this is a big time crap. have you ever heard of "I am boring of having sex?"

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some girl might tell u , sex only after marriage ...
and ya da ya da ya bla bla bla

get her into good ambiance , sweet talk her, treat her the best
start to give her erotic kisses and there you go with good foreplay

if she is not seeing someone else, definitely she is not enjoying having sex with you