You know you're ready for sex when you and you're partner are both legally old enough to consent, you both understand how conception occurs and how to prevent conception, you both know how to practice safer sex, and you've both talked about what you want out of sex.
You know because you simply feel ready and you can trust your partner, it's best to start with exploring each others bodies to gain trust, learn to relax with each other, and learn what turns each other on - it's best you know this from exploring your own body first.
when he is ready/ 16 or older
You are if you've never had sexual intercourse.
having what they think is their final sexual intercourse with their wives
When your boyfriend kisses you suddenly then you can kiss him back, but the next thing will be heavy petting that could lead to sexual intercourse so you need to let him know just how far you are willing to go. Never let anyone pressure you into sexual intercourse if you are not ready.
When he engages in sexual intercourse.
The same as sexual intercourse. If you don't know that term either, the Wikipedia article on "sexual intercourse" should give you a good overview.
Sexual intercourse.
That means that you refrain from sexual intercourse - in other words, that you don't have sexual intercourse.
If you haven't had sexual intercourse yet, then you're still a virgin.
Because you're having sexual intercourse with someone else.
by having sexual intercourse with a bear
During sexual intercourse between a man and a woman, the man places his penis inside the woman's vagina. This is known as vaginal intercourse. Sexual intercourse could refer to any type of sexual activity in which one partner penetrates another. Other "types" of sexual intercourse include anal intercourse and oral intercourse.