Women always have excuses when you want to have sex with them.
It goes like:
(1) Can't have sex with you tonight because I am too tired.
(2) Can't have sex with you tonight because I've got a headache.
(3) Can't have sex with you tonight because I am your sister!
While that was funny apart from (3) being pretty sick. I like to include everyone on the forum, so this is revised for Steve.
Men always have excuses when you want to have sex with them.
It goes like:
(1) Can't have sex with you tonight because I have no lube.
(2) Can't have sex with you tonight because I've got arse ache.
(3) Can't have sex with you tonight because I am your brother!