That all changed one spring day three years ago, when the Toronto native, who works at a New York-based international development organization, received a surprise telephone call from an old university friend.
"Do you want to be a dad?" asked Julie Murray, an English professor at Ottawa's Carleton University. Along with her lesbian partner, Lauren Gillingham, she was looking for a man who would donate more than just sperm: He would also be a parent.
Six months and many soul-searching discussions later, the trio - plus three lawyers - sealed the deal with a legal move being used by a growing number of unorthodox Canadian families: a contract.
"The parties believe they have a commonality of values about life and parenting," reads the 17-page legal document, signed on Sept. 16, 2004, by Mr. Irwin, Ms. Murray and Ms. Gillingham almost two years before their daughter, Helen, was born. "They choose each other."
Advances in reproductive technology and the growing number of same-sex parents are creating a long list of new family forms: two lesbian moms and two gay dads; two dads and a part-time mom, even men and women who met through personal ads for the sole purpose of having a baby.
Non-traditional families have created what many lawyers and human-rights experts say is the next frontier of family law: the legal recognition of families with multiple parents.
Many of these parents aren't waiting for a nod of recognition from the Canadian legal system, which was based on the two-parent, heterosexual family model. Even though the contracts are not legally binding, some parents are drawing up formal agreements that cement details ranging from which parents have primary custody to who has the child during holidays or weekends and who will have custody if one parent - or two - gets sick or dies.
"You end up having to discuss all your motivations," says Mr. Irwin, 40, who is deputy director of scholarships at the Open Society Institute, a foundation that helps to develop democratic civil societies around the world. "You look at it all with a fine-tooth comb."
Judith Stacey, a sociology professor at New York University, has studied many of these contracts in her research. Some start with loving preambles outlining the spirit with which the child was conceived. As much as 30 pages long, some give detailed accounts of financial obligations and how child care will be divided between three, or even four, parents.
One lesbian couple and gay man living in Los Angeles promised to buy a duplex together so their future son could sleep alternately on each side.
"They're quite inspirational and moving, often," Ms. Stacey says. "I wish that all parents would think ahead of what it means to become a parent."
Ms. Stacey is one of many legal and social experts who say co-parenting agreements play a valuable role in the formation of non-conventional families, which don't have mainstream models to look to as guides.
The contracts ensure that uncomfortable issues - death, breakups, new partners, relocation because of career, or a child's religious upbringing - are discussed before the child is born so that everyone is on the same page - particularly because the law may not be there to back them up.
To ensure Ms. Murray had a legal connection to Helen, Mr. Irwin promised to take his name off the birth certificate so that Ms. Murray, who is not biologically related to Helen, could adopt her and have custodial rights.
The family also agreed that Ms. Gillingham and Ms. Murray would consult with Mr. Irwin about major issues, such as how Helen is educated, but the women will have ultimate say as the child's primary caregivers.
Not everyone thinks contracts are a good idea.
"People say, 'This is about trust, and putting it on paper sort of demeans the process,' " says Chris Veldhoven, who runs parenting courses for gay, lesbian and transgender parents at a community centre in Toronto.
Others point out that once the child is born, even the most careful plans can go out the window.
"Imagine, if 15 years ago you had to sign a contract for how the next 20 years of your life were going to develop?" says André Chamberlain, a Crown prosecutor in Toronto who for 15 years has been raising his son with a lesbian couple according to verbal promises made to one another.
Mr. Chamberlain first met his son's biological mother at a patio café in Toronto's gay village, after she posted an ad in a local newspaper looking for a sperm donor who would also be involved, part-time, in the child's life. Now Mr. Chamberlain, his son and the two moms and their adopted daughter live 15 minutes apart, share Christmas and birthday celebrations and, sometimes, family road trips.
Legal experts point out that even with a contract, parents such as Mr. Chamberlain remain legally vulnerable. The contracts aren't binding, and serve as little more than a statement of intent in the eyes of the law should families be torn apart by arguments, death or circumstance.
In Canada, the child's best interest is paramount in all custodial decisions - making co-parenting agreements and contracts nearly meaningless.
For three-parent families, the non-biological parent is at particularly high risk of losing custodial rights. Without a name on the child's birth certificate or a genetic bond, they will not necessarily be recognized by the courts - regardless of whether they've been there through thick and thin: conception, birth, scraped knees and the first day of school.
"We're in limbo in terms of the legal structure," says Hilary Cook, a lawyer who serves as chair of legal issues for gay-rights organization Egale Canada. "No one really knows what a court may do."
Mr. Irwin sees nine-month-old Helen every six weeks. He either travels to Ottawa or the trio travel to Manhattan. That's all the time his job - and Helen's breastfeeding schedule - will allow; but when Helen is older he hopes they can hang out, perhaps go on vacations, on their own.
As for their contract, it's buried somewhere in Helen's Ottawa home. "It's probably alongside our mortgage or something," Ms. Murray jokes.
All three say they're happy they signed. But they also say they don't think they'll ever need to use it.
"This just makes it so legalistic and a bit stark," Mr. Irwin said recently, looking at the contract for the first time since Helen was born. "The reality is so much more complex, and so wonderful and so rich."