I'll keep it short; Lately I've not been wanting sex,

Would you be "offended" if she wanted to have sex with you? If anything she's going to think you're not attracted to her if you don't initiate. Just tell your partner that you want to have sex and then do it. I don't know where so many guys get this ridiculous idea that sex offends women.

You don't need rights in order to have feelings. However you have to communicate those feelings to your partner and realistically consider what it means to your relationship. Be direct and encourage her to ask questions rather than making assumptions. Otherwise it's really easy for her to jump from "doesn't want sex" to "doesn't find me attractive anymore." The conversation should follow a format something like

>you: briefly state how you're feeling
>her: questions for clarity
>you: your concerns about what it means for the relationship
>her: her input on what it means for the relationship
>you: your ideas about how you want this to resolve
>her: her feedback on those ideas

You should just tell her that you've not been in the mood lately. She should understand.

Everybody has periods like this. Not everybody wants sex all the time.

>>16945766
You seem to have misread.

OP said that he didn't want to have sex.

>>16945780
Ah I missed the "not", sorry. Please excuse my post.

If you're not in then mood, then just tell her. But be a good partner and at least get her off. That doesn't require you to be in the mood yourself, just some effort out of the love for your girlfriend.