I agree OP.
To everyone saying "seen worse on stag/hen does" a plane is a captive environment. You can't just walk away or turn a blind eye as it is right there in your face.
That is obviously something plenty of people would find offensive and reasonably something you might not want young girls/children to see. It doesn't make sense to allow people onto a plane displaying things that are likely to cause offence. It sets up potential for conflict and atmosphere on a plane which coupled with alcohol, confined space and plenty of people on the valium is a recipe for disaster.
I totally agree with the airline.
And I think it's a sad day when women would refer to themselves as "bitches".