Councillor Summers was expelled from the Green group on Brighton and Hove City Council last year after she voted against a motion that supported same sex marriage.
Mr Kirby has been a longstanding supporter of reform and voted in support of the Marriage (Same Sex Couples) Bill in the House of Commons last month.
The debate has been organised by Changing Attitude Sussex, which was set up by Dr Keith Sharpe.
The organisation says on its website: “Changing Attitude Sussex is committed to telling the truth about Christian teaching on homosexuality, and works for the full inclusion of lesbian, gay, bisexual and transgender people in every province of the Anglican Communion and more widely in all Christian churches.
“Our next meeting will be a public debate on gay marriage to be held in Brighton’s Gay Village on Friday 21 June.”
The same sex marriage bill, also referred to as the gay marriage or equal marriage bill, is next due to be debated in the House of Lords next Monday (17 June).
Changing Attitude Sussex said: “Changing Attitude Sussex will stage a debate on gay marriage on Friday 21 June at 8pm at St Mary’s Church on the corner of St James’s Street and Rock Gardens, Kemp Town, Brighton, in the heart of the gay village.
“The principal speakers will be Simon Kirby, MP for Kemptown, and Brighton councillor Christina Summers.
“Simon has been an outspoken longstanding committed supporter of equal marriage. Earlier this year he said: ‘I support equality because I believe the institution of marriage strengthens families and society and I think that the benefits of a legally defined marriage should not be excluded to anyone because of their sexuality.
“‘I will be doing my utmost to support the cause of equal marriage in Parliament in the weeks and months ahead.’
“Christina Summers is now an independent Brighton councillor after she was controversially expelled from the Green Party because of her opposition to same-sex marriage, based on her Christian convictions.
“During a council debate on the issue she declared: ‘When you touch marriage, you’re hitting at the very heart of God, and I have an enormous problem with that.’
“The supporting speaker for gay marriage will be the Rev David Page, and a colleague of Councillor Summers will also speak against (Dr Sharon James from the Coalition for Marriage).
“There will be an opportunity for audience members to ask questions and raise issues with the speakers.
“This is a very timely public debate. Everybody is welcome and entrance is free.”