I have once read an article saying, based on a conducted study, that children raised by lesbians turned out psychologically better than children raised by "traditional" parents. The study was conducted for a period of 25 years following closely children of lesbians who were born using artificial insemination. These children, compared to children raised in "traditional" homes, turned out to be happier, more social, less aggressive and manifested less anxiety. Although the study was funded by gay and lesbian rights groups, the head researcher insisted that the data were not altered in any way. To help reduce potential bias, all the measures were scored using quantitative/number measures.
I think LGBT community will, in general, agree with the conclusions and I think that the majority will not think in any way that there is any subjectivity to it. On the other hand, I am sure that while some will remain neutral, a big part of the “traditional” world will think differently with more or less polite comments.

This wasn't the only study and I didn't come across any information that could lead to any other type of conclusion. There were few surveys that stated that oftentimes children in the same-gender relationship are more likely to face social and emotional problems. Despite what many straight people think, there is no indication what-so-ever that there could be any danger or big difference for the child to have two moms, two dads or a mom and a dad. No matter of sexual orientation the big factors I see could be reduced to the parent's role, the society's acceptance and the child’s take on the situation:

1. Gay or straight, the parent has the highest role in a child's life. If the child is born in a straight relationship the parents have the role to raise them without having any prejudices. When the child is born in a gay relationship, the parents must do a bit more, meaning that they should first explain their child why he/she has two parents of the same gender while the majority of kids have one mom and one dad and also prepare them to understand things and help them build a strong self-confidence from an early age. Kids are adorable and sweet but they can be very cruel to each other. Being different will always put you in a spotlight. It is not always with negative meaning, but being a child of gay parents will mark you immediately, especially in countries or cities where there is still a lot of adversity or intolerance towards the gay community.
2. What do gay couples do? If they aren’t strong enough to come out do they decide never to have children? From what I have seen so far, yes, many couples do that. There are still those that decide that their wish of having children is bigger than the fears of what would people say. And they go through lots until they fulfill their wish. They love their baby like the most precious thing in the world. But not less and not more than other parents. They want their best for their child, same as same-gender parents. The only difference is how the society will treat them. At first, the other kids will ask their parents something similar to: “How come Ana has two moms and I have a mom and dad?” For my opinion here is a crucial moment of how their parents will react, what will they tell them, will they be tolerant or will they say immediately that this is not accepted, this is not normal, etc? Children will accept any answer they receive and they will form their opinion starting from that. They will also give weight to the opinion of their teachers. Hence it is also very important what the teachers have to say, how they will react when certain comments occur. If everything is done properly, the child born in a gay relationship has equal chances of having a happy life ahead as the others. They will not feel stigmatized from an early age. They will not even think that it is strange he has two moms or two dads. For them this will be as normal as seeing heterosexual parents. I believe that they will have a higher dose of acceptance as they are used from an early age to embrace diversity. As long as it isn't harmful, the best attitude is to accept and tolerate everything around you, no matter if you understand it or not.
3. What if the gay parents of the child are not “out” yet? They may be living in an area where their life will become miserable if they only tried to confess to the closest friends or family. But they want so much to have children. While for an M/M couple this is really hard to hide, for a F/F one I believe there is still a chance for this to happen without any comments from the closest people. It will not go without saying a lie or hiding few things here and there regarding their relationship, who the father is and where did he go. Later on, the child may never find out that his mother's friend is actually his second mom, but he will perceive her that way. He may not call her Mom but she will know it. There will be less stigmatization at school as he will only be marked as being a single-parent child. In any case, I am sure that he will be raised with almost the same approach as if his moms would be officially in a relationship. But will be a lot of "behind the doors" things for the parents.
4. Sure, some studies proved only good things but you can't take your decision based on that. What you can do is to make sure you have all it takes to bring on this world and raise a child that will have everything to make the best out of it. By this, I don't mean material things, though those they have their role too. You need to be psychologically and emotionally prepared. I am sure the child will not lack love. But there will be something more important at one moment. The child will ask who his Dad/ Mom is. They will feel the wish to know them. No matter how well you will explain things, how well the society will accept them, they will still feel that something is missing. The majority of kids, books, movies, games etc. are all referring to a Mom and a Dad.

Do you feel your life will not be complete without having children?
Are you ready to go through all good and bad?
Are you sure you have what it takes?
Yes? Then get ready before you embark the journey. Enjoy the parenthood. :-)
No? Make sure you are okay with the decision, have no regrets and enjoy your love life at maximum.