The odds of insemination are lower, because things like self-lubrication and uterine peristalsis (which make a big difference) aren't typically going to occur.

A quick Bing search found this:

A SINGLE act of rape may be more than twice as likely to make a woman pregnant as a single act of consensual sex.

[...]

The Gottschalls focused on 405 women who had suffered a single incidence of penile-vaginal rape at some point between the ages of 12 and 45. Of these, 6.4 per cent became pregnant. But that figure jumped to nearly 8 per cent when the researchers allowed for the women who'd been using birth control-US government statistics show that 1 in 5 of the women in the sample were likely to have been using the pill or an IUD.

To complete the comparison, the Gottschalls needed to know how many women in that age group get pregnant from one-night stands and other one-off acts of consensual sex. The answer-reported this year in a separate study by Allen Wilcox, head of the epidemiology branch of the National Institute of Environmental Health Sciences-was a mere 3.1 per cent. "It was surprising to see this margin of difference," says Jon Gottschall.