There's a 1913 Suffragette poster at the V&A museum in London, which I first saw on the wall of a brilliant feminist academic who tutored me at Cambridge. It has 10 simple, woodcut images illustrating the headings: "What a Woman may be, and yet not have the Vote", and "What a Man may have been, and yet not lose the Vote." The first five show women in the roles of mayor, nurse, mother, doctor, teacher and factory hand. The second five show men in the roles of convict, lunatic, proprietor of white slaves, unfit for service and drunkard. I thought of it as I watched the equal marriage debate in the House of Commons on Tuesday.

If we were to reissue the poster today, it might bear photographs illustrating "What heterosexuals may have been, and yet preach the sanctity of 'one-man-one-woman' marriage" and "What gay people may be, and yet not be allowed to marry." On Tuesday afternoon, gay and lesbian MPs stood up and spoke movingly about all they have been, and yet still may not marry: Mike Freer has been in a committed relationship for 21 years; Margot James has been living with her partner for 15; Angela Eagle has been in a civil partnership since 2008; Nick Herbert's constituents happily elected him as an openly gay MP, and he has been in a relationship for 13 years, and a civil partnership for three.

Chris Bryant held his civil partnership ceremony in the very Houses of Parliament where last week he had to explain with great patience to Roger Gale MP that comparing gay marriage with incest "is profoundly offensive – not only to a great many people in this country who have formed civil partnerships, but to quite a few people on both sides of this House".

Mr Gale, as it happens, was one of those illustrating the category: "Married three times and divorced twice" – but he can still speak out about the sanctity of marriage, and why Mr Bryant and his partner should be excluded from it. James Gray, the Tory MP for North Wiltshire, was next, representing the subgroup: "Left his wife of 26 years after having a 19-month affair with a married woman while his own wife was fighting cancer."

When the affair was revealed, the cuckolded husband wrote to his local newspaper: "The irony is that I will not reap the benefits of Mr Cameron's excellent family-based policy proposals because one of his own MPs has ripped my own family apart."

Also speaking against equal marriage was Adam Afriyie, who is divorced and married to another divorcee. The Tory blogger Iain Dale claims to know of two male MPs who are married to women, but having affairs with men. One can only wonder how they voted.

The difference between this and the suffrage debate is this: back in 1913, even the most vehement anti-suffragist didn't argue that giving women the vote would change the definition of voting. Equal marriage doesn't redefine marriage; it just lets all consenting adult couples do it. Let's hope that, in a few years, it will seem as obvious and unquestionable as letting all adults vote.

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