I can attest to Lisa's comment, about masturbating in front of someone else. I have done it. I've jacked off in private many times but there is nothing like doing it while someone else watches you.
For years I have had a thing going with one of my wife's nieces. We have never had intercourse but we do masturbate in front of each other and have done so many times.
I also have a niece and I talked her into masturbating with me and it was just as much a turn on as my wife's niece. Maybe even more so since we are blood relatives.
I met a woman in a hotel bar once when I was out of town on business and we ended up going to my room and we both masturbated.
I have also had a few photo shoots that I have done with women and some of them agreed to masturbate while I took photos of them. In every case I also masturbated while they watched me. Jacking off in front of a stranger is just as hot as doing it with someone you know.
It is a turn on to watch another person masturbate, but even more of a turn on to have someone watch you.
I do not know what the thrill is but taking your hard dick and playing with it while a woman watches you causes your dick to throb and the precum leak. And then when the release comes and you shoot your cum out your dick does not want to go down because it is so hot.
If you have never masturbated while someone else watched, you should try it. Man or woman, you will find it to be one of the most thrilling and hottest things you have ever done.