Does casual sex establish a marriage?

Question

• Do premarital sexual relations effect Kiddushin (betrothal/first step of Jewish marriage) if there is no intention of getting married?
  • If so, is the physical act itself determinative of whether or not there is a kinyan (transaction effecting Kiddushin)? (Let's say they would instead be just hugging and kissing: would that effect a kinyan also?)
• Does the Yichud (seclusion) that the couple is having effect Nisuin? What if there are no kosher witnesses around?

Answer 1

Premarital sexual intercourse is not a kinyan unless it is done with the intention of being koneh. Not only that, but the man must say as much. Rambam Hil Ishus 3:5 (my own translation):

ואם קידש בביאה אומר לה הרי את מקודשת לי או הרי את מאורסת לי או הרי את לי לאשה בבעילה זו וכל כיוצא בזה. ומתייחד עמה בפני שני עדים ובועלה

If he betroths (mekadesh) with sexual relations, he says "Behold, you are betrothed (mekudeshet) to me", or "Behold, you are married (me'ureset) to me", or "Behold, you are a wife to me with this intercourse", or anything of that sort. He secludes himself with her before two witnesses and copulates with her.

Now, the seclusion works because there are witnesses and because they know that his intention is for marriage. (See Maggid Mishna.) If there's no witnesses, there's no kiddushin.

As a side note, there is a separate prohibition against premarital sex. See Rambam Hil. Ishus 1:4.