I read this as she's either bi and not acting on same-sex attraction, which is not uncommon. (As also the reverse.)
OR
Even if she is, she is not interested in him being involved in that aspect, i.e. she's monogamous as far as sexual activity goes. Anymore than she might want him involved in any other activity of hers! (I saw some question to an advice column where the bloke insisted on going everywhere with his partner including on girls' nights out, which was clingy and creepy.)