Desexed dogs having sex?

I know this sounds crazy- and I am not a troll- this is a legitimate question.

I have two desexed dogs who are very much in love with each other. Nelson is 2.5 years old and was desexed at six months of age when he was adopted from the pound.

Dolly is 18 months old and was desexed at 15 months when she had a prolapse. She had never been in heat prior to this.

I got Dolly a couple of months ago, and the dogs are very attached to each other, they kiss each other and play together all of the time.

Weird thing is we have witnessed the dogs having sex.

First time I saw it I wasnt looking too closely- I thought Nelson was just 'humping' Dolly like a dominance thing, but my boyfriend caught them at it yesterday- and apparently there was penetration involved.

Is this normal? I didnt think dogs did this kind of thing, and I cant find any info. I have never heard of this happening. Has anybody else witnessed such an event?