Heterosexual couples are increasingly seeking to bypass what they see as discriminatory language in the Marriage Act, some wedding celebrants say, as the bill to allow same-sex marriage enters Federal Parliament.

Legally, the phrase "marriage, according to law in Australia, is the union of a man and a woman, to the exclusion of all others" must be read in the presence of at least two witnesses during the exchanging of wedding vows.

However, some celebrants say couples who consider that phrase discriminatory are finding ways of getting around it.

One told the ABC some couples were asking their guests to block their ears for that particular sentence, or following up the phrase with colourful passages such as "... but we think that is bullshit".

Bill Shorten on Monday introduced legislation into Federal Parliament to allow same-sex marriages and to recognise those entered into overseas.

Paul Voge, a former record store owner, officiated 95 weddings last year and said every couple wanted to add something personal after the compulsory vow.

"That really does suck," he said of the legal passage. "It's a real downer."

"Every single person says 'how can we not say that?' and 'is there something we can say after that'."

Mr Voge said while it was tempting to just add "but we think that's ridiculous" — or something more expletive-laden — after the phrase, he has instead written a formal passage that he offers his clients.

"As far as some celebrants go, I might be considered to be this weird kind of punk rock dude, and I would be the one to say "yeah, but that's bullshit, or ... that's ridiculous," he said.

"But I feel that kind of makes a comedy of it.

"To me, the solution would be say something that sounds exactly like [the legal passage], that proves the point to anyone, whether it's some younger couple, or whether it's grandma, or whether it's my mum — who I had in mind when I was writing this — [that] they can't really challenge it.

Paul Voge officiates a wedding. ( Supplied: Paul Voge )

"I would then say, 'it's important to Jack and Jill that they feel that marriage, like all expressions of love, cannot and should not be discriminated against by law'."

Joshua Withers, a celebrant in Brisbane, said one couple printed cards saying they wished marriage was available to everyone and left them on guests' seats, while others did the official ceremony privately and then had a homosexual friend officiate a mock — though more personal — ceremony.

Mr Withers said while most couples he worked with did not want to make the event too political, some felt the need to take a different tack because someone close to them was gay.

"Usually it's just that they are embarrassed that they can get married but their friends can't," he said.

Celebrants 'flirting with fire'

Celebrant Linda Brewer said that, although she hoped the law would change to allow same-sex marriage and had heard of celebrants being asked to tweak the legal wording, she would not want to meddle with the legal aspects of a ceremony and would not feel comfortable asking guests to block their ears.

"While the act is as it is, we have to abide by the rules," she said.

Keith Lammond from Australian Marriage Celebrants Incorporated, an association for professional celebrants, said celebrants would be "flirting with fire" if it appeared statements criticising the act were coming from them and not the couple.

"If we were to make derogatory statements about what we are supposed to say, the Attorney-General's department will frown on that," he said.

He said celebrants that seemed to show contempt for marriage in its current form "frankly don't deserve to be a celebrant".

Mr Lammond added that, in his experience, including a statement about marriage equality in the ceremony was uncommon.

"I've been doing this for 21 years and I can count on one hand the number of people who have asked me to make that statement."

When it is made, however, celebrants say guests notice it — and for Mr Voge, one response has stuck with him.

"A man, he must have been about 60, he came up to me and he asked me about that part that I said. And I was 'like, shit, this dude is going to have a go at me about the sanctity of marriage or whatever'.

"And he [said] 'my daughter, she is in a relationship with another woman, and I just thought that was a really good part — can you tell me what it is?

"I cut it out of the ceremony [book] that I had done and said 'here, take it, you can give it to her'."