Sex is both a physical and a spiritual act. Let's look at how it affects your luck in life.
Sex binds you and your partner spiritually. When you are bound like that, it means what affects him or her spiritually will also affect you.
In marriage, this means the good and the bad. A lot of men are progressing in life because of their wives. If you get a woman who has a lot of luck, you will automatically be a beneficiary.
The opposite side is also true. If the woman or man you married is very unlucky, it will affect you.
Outside marriage, sex only brings bad luck. This is because the good sides are only guaranteed by God Almighty. Since having sex outside marriage is not pleasing to God, you will not get luck even if the man or woman you slept with has a lot of luck.