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In England and Wales:-

In case you're wondering, Scotland and Northern Ireland are different countries, so I have no idea about their laws.

A bit of history

Until a few years ago, the age of consent for male homosexual sex was 21, also the age at which one becomes eligible to sit in the Commons. There is no clinically proven link between these two facts. In fact, both of these laws were introduced in the Victorian era, with the original age of consent for a girl being 12 years of age. The lowering of the homosexual age of consent was passed with opposition among Daily Mail readers and the Lords, as it was felt by a large section of the population that another large section of consenting adults should not have to commit sexual offences in their bedrooms for mutual gratification. It is reasonable to assume that the age of consent for girls was raised when it was recognised that no girl would be married before the age of 16, which is in fact prohibited by law now, as well.

The definition of "sex"

These definitions are for the purpose being charged with a sexual offence:
Sex with a girl

This is what you think it is, except that mere touching would not count. A girl under 16, but over the age of 12 is not legally barred from consenting to be touched. Of course, a man touching an unconsenting woman is sexual assault; a woman so touching is merely guilty of assault. Of course, any sexual contact with anyone under the age of 18 who does not in fact have capacity to consent is still actionable under various rape and child protection laws.

Male homosexual sex

In fact, this is confined to buggery.

Heterosexual sex with a boy

This is what you think it is, and may be actioned under various English child protection laws.