The artificial vagina seems like a weak point in the experiment. I don't think a real vagina where the tissues engorge and where the elastic mechanism is different would necessarily behave the same way.
Still: Finally! I got really sick and tired of evolutionary conjecture about this subject getting passed off as fact when no one could point to any experiments done on the subject.
ETA: Also- I forgot to mention foreskin. I know it's supposed to retract but I feel like the researchers should consider an uncircumcised penis for the purposes of evolutionary biology.