I'm A I'm a decent looking 25 year old lad who lives with his gf S who is a couple years older than me. We have a good sex life and our relationship is still strong. I've never even thought about cheating on her. A few months ago s had her friend who had moved out of town stay over for a long weekend. K arrived on the Friday she's about 5,6 long dark hair a size 10 34d tits lovely bum she has a bit of a Mediterranean look about her. We all get on great k can be a bit shy at times but we all have a laugh. The Friday eve we were all in the house a and k spent most of the eve having a catch up as I watched tv in the other room. It was the Saturday morning and s had got up to go to her hair appointment thinking k had gone with her and I would have the house to myself for a bit I got out of bed and walked to the bathroom naked not closing the door behind me. I was in the shower in a world of my own my cock semi hard when all of a sudden k walked in. Stood facing her in the shock I forgot to cover myself I noticed her eyes fixed on my cock for a second before looking at me and apologising and walking out. I finished up and went downstairs where k was in the kitchen making a drink. I'm so sorry about that she said I didn't realise you were in a said you normally play football on the weekend. That's tomorrow I said it's ok don't worry about it. We made a joke about it and went in the living room chatting. She was talking about her recent break up then saying s was a lucky girl to have a nice younger bf. I blushed she continued she's told me a lot about you said with a cheeky smile. I just laughed it off unsure what to say I said to her I'm sure she will find someone else. She said she didn't wanna rush and just have a bit of fun she was enjoying the single life again. Then she was saying that she just wanted a fuck buddy really no strings. At this point for some reason I could feel my cock beginning to grow as the convo got more sexual. She was saying how she got bored of her ex and he didn't pay her much attention. Shame on him I told her your a good looking woman. The convo got more flirty with k saying how lucky a was how I have a good body and that no wonder shes always happy giving a knowing nod that's she's seen my semi. Then she started talking about a site she had joined up to meet men for a bit of fun. She had logged in to her profile in her phone an was showing me. It was a swinging site but she just wanted to meet guys. I was browsing her profile when I clicked on her pics she quickly took hold of the phone and said naughty you can't see them, you've seen me I replied. Unsure she said ok but don't tell s I've shown you don't worry not gonna say aslong as you don't mention the bathroom incident, deal she agreed. Looking at her pics she had a few nice ones of her in underwear but nothing to explicit. She was saying she wanted more pics but couldn't do them herself. For some reason I said I would take some if she wanted. Unsure she said we can't I said don't worry s won't be back for another hour or so at least will only take a couple of mins. What sort of pics you want. She explained guys always ask to see pics of her bent over or from behind ect. Ok do it in the bedroom I said as I don't want s to click they were taken here the bedroom had a plain white wall. K said ok I'll go get ready about to mins later she called me I went up with her phone. Standing at the bedroom door in a black bra and tiny black thong is this ok yo think yeah looking good I replied. Nervously she turned round her ass was perfect my cock beginning to grow. I took a couple of shots of her leaning against the wall sticking her ass out then a couple of her bent over on all fours on the bed. By now I was rock hard I passed k the phone to have a look at the pics she liked them and she noticed my cock straining against my bottoms. Sorry I said I can't help it you have a great body. She giggled my days it looks huge how big is it. About 7 I told her but my cock is thick that look bigger when straining my bottoms. She giggled your naughty I told her I'm sorry but I do really need to cum. I walked to the bathroom pulling my bottoms down I just began to wank near the sink she soon followed me at first she watched for about 20 secs before saying can I help. I told her to lean against the wall and let me see that ass. She did as I asked I walked up behind her and began feeling her firm arse cheeks I dropped to my knees I ran my finger underneath the material of he thong between her legs she was really wet. I pulled her thong down her pussy looked so good I just kissed it she gasped as I then began to lick her. I spread her arse licking her pussy even her tight arsehole she was moaning a lot getting wetter by the second. I then told her to bend over the bath standing behind we I slipped every inch of my cock inside her. Her pussy felt so good I had soon built up a rhythm and was really pounding her from behind. Oh god fuck me she gasped as I was in full flow I was also groaning. After about two mins of really fucking her. Could feel myself about to cum not wanting to cum inside my girls mate I pulled my cock out and shot about 4 big squirts of cum all over her bum my legs nearly holding me up as I came so hard. Standing up and turning round wow k said I needed that I haven't had sex for weeks. We soon both got ourselfs together and both showered and got dresses knowing s would be back soon. She arrived back about 20 mins after we both sorted ourselfs out. We carried on as if nothing had happened. S is unaware of this I do feel guilty of what I done but can't get over the thrill of how much of a naughty slut her mate is.