This topic is fairly vague and confusing.
What we have here is a complement of two different subjects thrown into one.
1.) Same sex marriage
2.) Being a parent
Topic 1.) Same-sex marriage:
-You can say anything you want about a man loving another man, or a woman loving another woman. In the world, we are all equal and everyone bleeds red. There is nothing wrong with someone falling in love with their same sex. It is no different than falling in love with your dog, or your cat, or your favorite spaghetti meal. You can rant to me all you want about what the Bible says about same-sex marriages and how it is morally wrong, but everything we do would then be a moral disaster if we follow the laws of the Bible.
Topic 2.) Being a parent:
-Not every human being in this world (man or woman) is fit to be a parent. Some to most men or women refuse to be parents just because they do not want to be held responsible for the caring of another person, and choose to live their lives the way they want. I know plenty of people who are retired, and have absolutely no children just because they didn't want the responsibility, and they were able to be more successful in their lives. On the other hand, people who are not FIT to be parents will end up having children, and ultimately end up ruining their life by making poor decisions, and abusing their responsibilities of taking care of another human being. I also know plenty of people who are NOT fit to take care of children, and live in poverty. (Ex. I know a girl named Kate who has three children with three different fathers. She lives in a busted apartment that is so horrible, all it would take is one person with their head on their shoulders to walk in the door, and see the maggots on the floor, and dirty diapers laying everywhere, and call DCFS to get those kids to a better place. Not a very healthy living environment.)
How these topics should NOT be mixed:
These two topics on their own raise questions whether anyone is a valued parent. As same-sex marriages go, I see no problems in this as I have plenty of gay friends (men and women) and they are just as normal, if not, more friendly and normal, than a heterosexual like myself.
But, to go onto your topic of whether homosextuals/lesbians should be able to adopt/create children themselves, Yes. I am a firm believer that if you are a responsible enough adult that you can love whoever you want, and if you can take care of a child, there is no issue why your sexuality should effect why you can or cannot take in a child.