There is an objective answer. Evolutionary psychology explains it like this: (These 'fears' are not conscious- you are not aware of why these feelings occur, they occur within your subconscious)
Contraception has not always been around (last 60 yrs etc). For the last 100 000 years, having sex pretty much meant having a kid. If you fuck a guy who isn't committed to 'sticking around', your ability to survive was virtually zero (women pretty much needed men in those days- work/bring food etc). So from an anthropological perspective, women have evolved with this fear of sex- not the act in itself, but the fear of the guy not committing. I like to deem this "last minute resistance" (LMR). The way to combat it guys is to demonstrate from early on that you will be there for the girl basically, show that she occupies some of your thoughts- a simple "Hey I was thinking about you today...blah blah" over the course of your courtship (dating) period should do the trick. That's of course if you want to sleep around. Additionally, guys find it hard to imagine, but from a subconscious perspective, imagine sleeping with someone who could physically kill you? This sounds ridiculous, but men being much physically stronger can be a threat to a women's survival if he was to go crazy (this is more an evolved fear as opposed to today's society with laws etc).
Anyway, on the flip side guys fear approaching women (lets face it, even if your a "natural", most guys get that anxiety feeling before approaching a random attractive women). Why? This is an evolutionary fear as well. Back when we lived in tribes of 50-150 ppl, if you approached the wrong girl (as in one preselected by the tribal leader) you would be effectively killed or exiled- which meant, you would not go on to spread your genes or mate (you'd also never have sex again- a huge desire for all males). So, our emotional circuitry has once again evolved over this time period to give us a hardwired fear- one which although not as relevant today (with 6 billion ppl in the world, way less scarcity), it is still there in order to protect our survival.