I am wondering, if two women sleeping in the same bed, but not in a sexual
way, Are they committing a sin?

I can not see how this would be wrong.

According to the Bible, is having a girlfriend at the age of 15 good or bad?

The Bible doesn't talk about dating or the age that one should date or marry. Dating is a serious thing contrary to popular opinion. I believe one should not date until they are mature enough to understand how to treat
their date with respect and how to avoid bad situations. For some this age may be 15 for others it may by 40. The Bible however never specifies an age.

I would like to know if the Bible supports the idea of "sex for pleasure"? Is sex for reproduction only, or for pleasure also? Is sex for pleasure sin? According to nature, sex is meant for reproduction only, but I would like to know if the Bible supports that idea.

The sexual union is the most intimate mankind knows. It was clearly intended by God that this would be a part of manís makeup, for He made them male and female, Genesis 1:27. The sexual union was involved in Godís first command to man, "be fruitful, and multiply, and replenish the earth," Genesis 1:28. (Though the word replenish is used in the King James Version, the Hebrew word really means to fill. No one had lived on the earth prior to Adamís creation.) When God finished His creation and looked at everything, He said, "It is very good," Genesis 1:31. The only thing that God saw that was not good was that man was alone, Genesis 2:18. Obviously in Godís first command to Adam and Eve there is the idea of the sexual union for the purpose of reproduction. 

However, other passages indicate very clearly that God gave sex for the purpose of joy as well. Proverbs 5:18-19 would indicate this. In 1 Corinthians 7:3-5 the Bible says that mates should not withhold themselves from their partner unless it is for a short time and for spiritual reasons. Otherwise it would be considered defrauding the other partner. The Song of Solomon speaks poetically of love and physical desire outside the context of procreation. God gave to mankind the sexual urge, but it is for marriage only. It is for pleasure as well as procreation. Therefore couples should feel free to enjoy this gift from God.