I agree with MoreThanRhetoric. Don't bite off more than you can chew, as my old grandmother would say. Grandfather kept a loaded 12 gauge and felt if he couldn't hit them in five shots they were welcome to shovel him under.
Get a license to carry, learn to shoot and pack your gat. Objecting to public sex (which might be rape) while displaying that you're armed will cause the attacker(s) to run, even if they are also armed. Criminals don't want a fight - they want victims. After the dust settles, call your attorney. Let someone else call the police.
Mind you, even if you save a life or stop a crime there will be ramifications for your civic mindedness. If you're armed you'll be vilified by the police and commercial media. The police will view you as an armed and dangerous criminal. You'll be beaten by the police, your home will be ransacked for 'evidence' and your family terrorized by the SWAT team as they serve one of their infamous 'no knock' warrants. If you have a dog, the SWAT team will kill it and leave the remains for you to bury. You'll spend one or more nights in jail while the police 'sort things out'.
Then there's likely to be a civil suit. The rapist is actually an honor student, choir boy and boy scout who was framed. Or something. The family - baby momma, baby daddy and notable relatives from all sides - are hopelessly distraught over the loss of their little angel who was destined for greatness. So, they want reparations. A few million ought to be a good beginning. Commercial media will pick up on the trial, so you can be the six o'clock news will have a clip of some prime Shaniqua Theater with you playing Snidely Whiplash.
I guess you have to ask yourself if all that suffering which you will endure is worth the life of a total stranger. Personally, I haven't come up with an answer to that one. In spite of this, I believe that I would stop the car, get out and put a stop to sex on the sidewalk. At least I believe I would.