Ged Killen, Labour MP from Glasgow, is married to a man from Northern Ireland.
During a speech at the annual Amnesty International Pride lecture in Belfast on Thursday he said:
"Make no mistake. Same-sex marriage in Northern Ireland will happen.
"Those who stand opposed risk alienating a generation.
"The children of future married same-sex couples, their friends and their families will not forgive nor forget those who stood in the way of equality."
A spokesperson for Iona Institute Northern Ireland said she disagrees with the comments and said it's an example of "very damaging information" as to why people are opposing the extension of same sex marriage to NI.
Tracy Harkin told Premier News Hour: "This is not an equality issue.
"This is whether or not we still need civil marriage as a social intuition and for the good of children, and in particular I think we do. It's been very frustrating to hear these constant accusations of homophobia or being anti-equality and I think that needs to stop."
Northern Ireland is the only part of the UK in which same-sex marriage has not been legalised due to opposition from its largest party, the DUP.
While Harkin rejected Killen's opinion, she acknowledged that the gay marriage debate in NI has had a negative effect on those who support it.
"That's an awful message to be hearing because I think it further marginalises an already marginalised community."
She called on the Christian community to better at articulating the reasons why same-sex marriage would be a bad idea,
"Children should have a recognisable mother and father that should be involved in their upbringing and that's the real reason why marriage as a social institution exists.
"Moral understanding shouldn't come into this discussion.
"In the public arena we're talking about civil marriage, so as Christians we need to focus on the common good of marriage. People don't care if they're not Christians what our theology is, but they do care about the common good and how it effects children. "
Stay up to date with the latest news stories from a Christian perspective. Sign up to our daily newsletter and receive more stories like this straight to your inbox every morning.
