Unfortunately, that is not the case. Stallman is absolutely a defender of having sex with children.

Richard Stallman on paedophilia:

"The nominee is quoted as saying that if the choice of a sexual partner were protected by the Constitution, 'prostitution, adultery, necrophilia, bestiality, possession of child pornography, and even incest and pedophilia' also would be. He is probably mistaken, legally--but that is unfortunate. All of these acts should be legal as long as no one is coerced. They are illegal only because of prejudice and narrowmindedness."

RMS on June 28th, 2003

"I think that everyone age 14 or above ought to take part in sex, though not indiscriminately. (Some people are ready earlier.)

RMS on 25th May, 2003

To be fair for that one, he doesn't specify whether 14 or under is fine for an adult to have sex with. It's certainly possible to interpret this as child-child relations only, but given his other comments where he says adult-child sex is fine, I decided to include this one.

"I am skeptical of the claim that voluntarily pedophilia harms children. The arguments that it causes harm seem to be based on cases which aren't voluntary, which are then stretched by parents who are horrified by the idea that their little baby is maturing."

RMS on June 5th, 2006

"There is little evidence to justify the widespread assumption that willing participation in pedophilia hurts children."

RMS on Jan 4th, 2013

I understand that Stallman has excellent views on liberty in software, and he's made enormous contributions to FOSS. But that does not necessarily mean he's a good person or that all of his views are good ones. People are flawed. IMO his views on the morality of having sex with children aren't good ones, but I recognise that I agree with him in other ways.