heard my mum having sex...

I can't believe the attitudes of people in this thread, as if the notion that your parents have sex is alien and disgusting...

It's pretty understandable that you're a little freaked out by it, and you frustrated about the fact she had a guy over effectively behind your back, but you have to ask yourself.... would you really expect her to be open about it? She's not going to ask you to leave the house, or say to you "I'm getting with this guy tonight, you don't mind do you?" And as for the fact your Dad pays the mortgage... yes, it's his house, but as long as he doesn't mind her living there, what's it to him what she gets up to as long as she's not causing damage or doing anything illegal? You can't really let someone live in your house under the condition that they're not allowed to have sex.

Maybe I'm strange, but I understand why you're angry. If she is doing nothing to pay the house off, then she needs to contribute if she's going to **** new guys without the common courtesy to at least introduce you two first. Sounds like she needs to grow the **** up and take some responsibility.

It's not the idea of parents having sex - it's the whole concept of her mum bringing a guy over secretly, having audible sex with TC nearby and probably being aware that TC would most likely be hurt and disturbed by it--both because it's with someone who isn't TC's real dad and because it's still disgusting to hear other people having sex even if they're not your parents--so it isn't just because of the thought of parents having sex is disgusting. It's just utterly disrespectful, emphasised by the fact that it isn't even her house.

That's utterly ridiculous. My Granddad had a builder mate of his over to pick up a tool the other day. I was fine with that. Would that mean I wouldn't have a problem with watching my Granddad having sex?

What makes you think that she does not contribute?
She might pay the bills? TV license? council tax? shopping? clothes? etc

my dad also pays my maintenance

First world problems.

I'm really confused by your comments about your dad paying the mortgage. For as long as I've been an adult, I've never considered it important to get permission of the property owner/bill-payer before having sex in a house that I am residing in.

If you're visiting someone and sneak in an extra guest, that might be questionable. But when you're living somewhere it seems only reasonable that you shouldn't have to ask permission before doing everyday things like having sex.