Remus Hamza, 40, is alleged to have attacked the woman near Cardiff University as she walked home alone on 20 September last year.
The prosecution at Newport Crown Court has alleged she was so drunk she could not have consented to sex.
The defendant, who is Romanian and lives in Cardiff, denies rape.
Mr Hamza told Newport Crown Court she smiled and "kissed him back" near the Civic Centre but did not say anything.
Speaking through an interpreter, Mr Hamza said: "This girl was looking at me, laughing at me, smiling and I thought she was smiling at me because she liked me and I just grabbed her backside.
"She just smiled at me and laughed... she came with me without me having to force her in any way.
"If she didn't want to have sex with me that night she could have just have said no."
He admitted the woman was intoxicated but claimed she had been "aware of herself".
He said he didn't tell police about the "consensual sex" in his first interview because he was embarrassed and had been advised by a solicitor to give no comment.
The court was told Mr Hamza pleaded guilty to exposure in a similar area in 2014.

The lab - which is not being named - is run by UK personnel, not the US military, said Colonel Steve Warren.
"Another lot came up positive and that lot had sent samples to the UK and Massachusetts," the colonel said, referring to the deadly bacteria.
It brings to 68 the number of labs involved in the anthrax scare from the US, including four foreign countries.
A number of US military facilities for the past decade have shipped inactivated anthrax samples to labs around the world to help their research into biological warfare.
But in recent weeks, it has come to light that some suspect samples were not properly de-activated before being posted.
After Tuesday's briefing, Col Warren told the BBC they could not be sure the UK received the live samples but it was sent samples from a larger batch - Lot 5 - in which some of the bacteria was found.
Staff members at some of the labs have been treated for anthrax exposure as a precaution, but no-one has fallen ill.
In addition to labs around the US, suspect samples were sent to facilities in Australia, Canada, South Korea and the UK.
Experts in bio-safety have heavily criticised the lapse and called for improved precautions.
Symptoms of anthrax exposure include skin ulcers, nausea, vomiting and fever, and can cause death if untreated.
The military has ordered all of its labs that have previously received inactive anthrax samples to test them. In addition it is advising all labs to cease working with these samples until told otherwise.
The Pentagon has maintained there is no risk to the general public.
What is anthrax?
Anthrax bacteria live primarily in inactive spores and are found naturally in the soil, but people can ingest or inhale spores, which can make the anthrax active.
Not all will become ill when exposed to anthrax but left untreated, anthrax illness can turn very serious or lead to death.
A blood culture test is used to confirm anthrax exposure.
Anthrax entered the US national consciousness in 2001, when shortly after the 9/11 attacks, letters containing powdered anthrax arrived at news organisations and the offices of US senators. Twenty-two people were affected and of those, five people died.
Sources: FBI Anthrax investigation; CDC, San Francisco Department of Public Health
A man accused of raping a 20-year-old woman in Cardiff during fresher's week said she "could have just have said no" to his advances, a court has heard.
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A lab in the UK may have received a sample of live anthrax from the Pentagon in 2007, say US officials.