i guess a mixture of several things including some of the things already mentioned. the first time i watched a girlfriend being fucked by a BBC, but it was not just the sight of them fucking unexpectedly, but the way the whole night went, we had been out and just drinking, nothing had ever gone on before in our sex lives, we had been together about 12 mths so still in the honeymoon period, we ended up in a local blues/reggae party club, got chatting to a fella who we clicked with, no flirting or nothing just banter, as the night went on she wanted to dance, i dont, so they did, watching her at ease and not a care in the world turned me on. after a break, he went to the bar, i made some joke about it being true about size, she tried to play it down thinking i was going to start moaning, and was pleasantly surprised when i said i was not jealous and the fact she turned other men on turned me on, so just enjoy yourself. We invited him back to our flat for a smoke and got in a taxi, i sat in the front, after about 5 mins i turned round to talk but they were busy kissing, the taxi driver was doing his best to pretend not to notice, so i turned back and just chatted to him, and they carried on, i had a raging hard on, did not understand the sexual arousal of what was happening to me, i turned back again and her hand had his cock whilst they were passionately kissing got to ours and he was very apologetic, i played it down and made a joke about her going a bit wild when *******, we got in to ours and rolled a smoke, little music and poured a *******, after about half hour i feigned drunkenness, told them to carry on and went to bed. lying there staining my ears to hopefully hear any tell tales moans but could not hear a thing but still hard as fuck i slowly inched down the hall and viewed through the crack in the door and the site was my girlfriend on her back legs spread high in the air and the bull in between her legs pounding her pussy, i was that turned on by the site of the contrast of skin, the stretching of her pussy, it took me 5 minutes to be able to move, they had not heard me above the noise they were now making, and i moved withing a foot of them and just stood there and watched, i ejaculated with out any manual help lol he got a bit concerned when he realised i was there but i just said carry on if thats what she wants and i was ok with it as long as everyone else was. we ended up both fucking her for the next 4 hrs we fell asleep then i was woken because he started fucking her from behind in the spoons position and he was non stop and must of cum in her 4 times. that was the start of my journey into cuckold