Sex before marriage is good, but sex after marriage is even better.
There are some things in life you shouldn't invest in without test driving it first.
Because in either case, if it all goes to sh-t after you've invested so much of your time, then you will be miserable and in many cases, there is no coming back from that.
If you wouldn't buy a car without test driving it first to see if it really is a good fit (no pun intended) then you shouldn't do the same with a person you are willing to spend the rest of your life with.
Sexual incompatibility is a very serious issue and most of the time the problems from it are not isolated to the body parts involved. It can have lasting consequences on the couple mentally, emotionally as well as sexually and when that happens, intimacy will suffer as a result.
As Dreadlord pointed out, safety is important, and just because you agreed to have sex with them doesn't mean that they, or you are doing so without boundaries or that you have to be their personal porn star or sex slave. You just have to have enough self-respect and self-control to set some boundaries and with NO exceptions.
For example, on 12/15 of this year, my husband and I will have been together for 13 years, and we were together for exactly 10 years when we got married in 2010. Although we had been living together and were having sex, there were certain privileges that were reserved only for my husband, and there were even some that were reserved only for my fiancé.
So whether we were together for 10 years or 20 more after that, there were certain things that I was not willing to compromise on because weren't married or engaged, and there were NO exceptions. Also, because we made the decision to live together first and also have sex throughout our relationship, there would have been no better way for us to know exactly how compatible we really are with each other.
It has actually made use closer as well, because to me, he isn't just my husband, but he's also my best friend and he feels the same way about me as well.
So my views on this are very similar to the idea of living together before marriage as well, since that would be the other thing that people should do before investing the rest of their lives with someone.
Entering this kind of commitment without any kind of litmus test for compatibility is foolish and a recipe for disaster. At least by having sex with them and/or living with them first, you will be making your investment into this person with eyes wide open and your judgment won't be affected as much as someone who did neither.
This is because, you'd be seeing them at their best as well as at their worst, complete with flaws and all, based on what you've seen, you can make a more informed decision and you can decide once and for all whether or not you'd want to be with them, compared to someone who has only seen the facade that the other person has put up whenever they are out on dates.
Oh, and a quick note about Michael Douglas, he is an excellent actor, but he is a freak, and this isn't because he is a sex addict. Now that i think of it, there isn't a movie of his that I've seen that didnt involve him having sex. So using him as proof against premarital sex is just as bad as using Charlie Sheen who collects porn stars like many kids collect Pokemon cards, and actually out-screwed Ron Jeremy, who IS a porn star or John Holmes, a porn star who died of AIDS in the 80's.