STDs the name speaks for itself. If a gay person has sex with the same person and that person has no STD then nothing will happen, but if a gay person has sex with everybody who wants... well then the risk increases, but the same rule applies for straight people.
When it comes to abolishing marriage completely (I've been married before, btw), well I'm not too sure about it. Especially when children are involved you will still need some rules in which things are set up, and what does happen to goods involved if one of the couple members dies? Marriage or no marriage some things will have to set up by a kind of binding contract anyway or the consequences may be horrible. So in the end.... The idea sounds nice, but when thinking things through, I'm not sure it'll work...