No accepted explanation for female orgasm

Carl Zimmer writes in the NY Times:

For biologists, few phenomena are as mysterious as the female orgasm. ...

For decades, researchers have put forward theories, but none are widely accepted. ...

The male orgasm has never caused much of a stir among evolutionary biologists. The pleasure is precisely linked to ejaculation, the most important step in passing on a male’s genes to the next generation. That pleasure encourages men to deliver more sperm, which is evolutionarily advantageous.

For women, the evolutionary path is harder to figure out. The muscle contractions that occur during an orgasm are not essential for a woman to become pregnant. And while most men can experience an orgasm during sex, it’s less reliable for women. ...

Still, a number of scientists suspect that the female orgasm serves some biological function favored by natural selection. They just need to figure out what it is.

Why is this so mysterious?

Assume that evolution is selecting traits that maximize grandkids. The male orgasm makes sense because it allows men to inseminate more women.

If women never had orgasms, they might not want sexual relations at all. If women always had orgasms quickly and easily, then they might terminate the copulation before the man ejaculates, and they might be less likely to seek out alpha males with higher quality genes.

It seems to me that women will have more grandkids if they are sexually frustrated most of the time. The orgasms have to be good enuf for them to want them, and rare enuf for them to be not too satisfied.

This same reasoning would not apply to men because have to ejaculate for pregnancy to be possible, but women can get pregnant without orgasms.

I am just speculating, but this seems like the obvious explanation. Any other explanation should be compared to this obvious explanation, but this article does not even mention it.

The article does mention the other obvious explanation:

Dr. Lloyd thinks the best explanation for the female orgasm is that it hasn’t served any evolutionary purpose at all. It’s nothing more than the byproduct of the development of the male orgasm. The orgasm is to women, she believes, as nipples are to men.

Possible, but that does not explain female orgasm frequency. Male nipples are not partially functional, they have no function.