Tory MP Nigel Evans groped a young man in a House of Commons bar, a court heard.
The complainant, one of seven young men the MP is said to have sexually assaulted, had been invited by Evans to the Strangers Bar at the House of Commons one evening in May or June 2010.
The complainant said he was introduced to the MP who was standing in a group of people, including former MP Lembit Opik.
Prosecutor Mark Heywood QC asked him what then happened.
The complainant said: ''We were chatting and Nigel reached out and grabbed my genitals, basically. Going in for a cup, I guess.
''At first I did not notice it but then, when you get the feel, you pull away.
''That is when I brushed it (his hand) away and said 'No, that's not cool'.''
He told the jury that Evans then walked away as if nothing had happened. The complainant immediately went on to tell his close friend, also in the bar, about what he said had just happened.
''I went to speak to (his friend) and had a bit of a giggle, to be honest,'' he said.
''It was a funny situation. I'm a straight male but that sort of thing should not happen but occasionally it does happen, so you have to be jovial about it.''
When it was put to him by Peter Wright QC, defending, that he made made the incident up, he said: "He cupped my genitals. It is building up a case where he should not be in a position of power where stuff like this happens and is swept under the carpet."
The complainant said no one in the bar appeared to have witnessed the alleged incident and he made no mention of it to anyone else on the night.
The court heard that he and some of the group went on to another bar, along with Evans.
He said he was ''tipsy'' when he met Evans - ''three on a scale of one to 10'', whereas the MP was ''five or six''.
But the complainant told the court that he "threw a sickie" at work the next day because of the amount of alcohol he had consumed during the evening.
He also admitted making a remark to Speaker John Bercow at the Palace of Westminster before the group moved on to another bar. Evans asked Mr Bercow if he wanted to join them for a drink but was told he was sitting as Speaker the next morning, the court heard.
The complainant then said to the Speaker: ''Don't be a c***. Come and join us for a drink.''
The complainant told the jury: ''I had had a few drinks. He handled it rather well, considering.''
Former MP Lembit Opik told a court there was a "not very savoury" exchange between himself and the complainant when he got to the bar.
"I went to buy a round of drinks, inviting the people in Nigel Evans group to tell me who wanted a drink.
"The gentleman in question I believe said something quite offensive because I had forgotten to get him a drink.
"He called me a fucking dickhead. I responded by simply getting the drink he wanted and making a note not to consort with that individual again.
"I'm not a confrontational person. I did get him a drink, gave him his drink, stood around and rejoined my group."
Mr Opik, the former Liberal Democrat MP until losing his seat in 2010, said he thought the young man's behaviour was "entirely inappropriate" and no one found it funny.
"I don't think anyone in their right mind could think I would be entertained by that behaviour.
"My judgement was that he was playing to the gallery, if nobody else had been listening he probably would not have said it.
"I didn't think anyone found it funny, how could it be funny?"
Mr Opik said he did not see any physical contact between the young man and Evans while he was present at the bar.
Evans denies one rape, two indecent assaults and six sexual assaults said to have taken place on various dates between 2002 and last year.
Former Coronation Street actress Vicky Entwistle - understood to be friend of the defendant - attended court today and sat in the public gallery.