I think you need to take the stripper out of the equation and try to think about how you'd react if it was an activity that was considered more universally neutral. To clarify, is this something that just doesn't appeal to you, or is it something that you specifically find off-putting?
I absolutely don't think anybody should feel obligated to do something (or help pay for a gift) if they don't want to, but I also think that in some cases, it's gracious to do something that you aren't particularly enthused about in order to support loved ones. Basically, if your feelings are more "meh" (neutral) than "ew!" (negative), I'd consider doing a grin-and-bear-it.
If the thought of watching or helping pay for a strip performance specifically makes you uncomfortable or goes against your values, I'd either skip the party altogether or say that you'll have to leave early, end stop.As Slartibartfast said, it can be tricky to accept only part of an invitation, especially if your grounds for rejecting that part are that you find it objectionable. It may be possible, but it requires a certain amount of tact. If you do want to attend the whole party minus the stripper, I don't think you should make excuses. People will often try to find a workaround. Just say, "You know, DH and I aren't really comfortable with this particular activity, so we'd like to sit that particular part of the festivities out. Can we go on a beer run/relieve the babysitter then?"