Wouldn't have happened if any of the 3 fully grown men had a gun.
This has been postulated, but even a cursory analysis of the mechanics of sex show that it is not true.
Consider the effectiveness of the underside of a penis in withdrawing semen during intercourse. Assuming a male-on-top position, semen will simply not be drawn out of the vagina. The head of a penis does not make an effective scoop. Compare the underside of the head to the sides - that ridge that they claim that does the scooping would be utterly ineffective when you have gravity draining semen to the underside of the penis.
Consider the fact that the typical thrusting motion does not involve pulling out completely - there would be little to no removal of a competitor's semen.
Consider the fact that even if it were an effective scoop and removal system, for it to be effective in preventing pregnancy from resulting (from the prior mates' session), the subsequent sex would have to occur immediately after.
The shape has infinitely more to do with the mechanics of insertion and the foreskin.
The thrusting motion has infinitely more to do with the mechanics of gettin' preggers - deeper penetration is beneficial. Larger testicles are beneficial too (more semen). Why did humans tend toward larger penises and not larger testicles? Because we walk upright. Depth of penetration is more effective than volume of semen when you're walking more upright. This is why humans like big dicks, this is why humans have a much more exaggerated thrusting motion compared to other animals and primates.