Is it ok to grab another girl’s butt when I have a girlfriend?

I have a girlfriend but I’m friends with a girl called Ellie who has a really nice butt and loves having her butt grabbed. I’ve discussed with Ellie whether my girlfriend would mind if I grabbed Her butt and I’m not sure if I should discuss it or not, or if I should just grab Ellie’s butt and not tell my girlfriend at all. It wouldn’t be sexual at all, just mainly a way of boosting Ellie’s confidence. Any ideas on what I should do?
Is it ok to grab another girl’s butt when I have a girlfriend?
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