for example in countries where women outnumber men, if every man married 1 woman there will be so many women unmarried. now would you rather have these women live a life of loneliness without a spouse? or allowing them to be a 2nd wife?
and the reason why women are not allowed to have more than one husband because there will be confusion on who the father of her children are.yes they have DNA test and all. but the husband would have to wait 9 months to find out which one of them is the father. so how could he father enjoy experience the of the pregnancy of his wife when he's not even sure if the child in her belly is his.

now,i personally do not think i could handle my husband having another wife. but having said that i would never put down a woman who was in a polygamous situation nor would i look down on a man that wanted more than one wife.

nib said "well.. ok i am single i dont have a gf. i want a relation and sex.. does islam has a solution for this??"

yes Islam has a solution for that. get married. one of the objectives for marriage is to satisfy one's desires. whether you are a man or woman. same rules apply to both genders. of course marriage is all about sex. but this is one of the objectives of marriage.
studies have shown that good marriages often have good sexual relations involved.and bad marriages the couples often don't have bad sexual relations

there is also another solution if you don't want to get married. the prophet Muhammad (pbuh) said to fast.as it decreases one's desire. so if you don't want to get married. then fast