I know my views and experience are strange and foreign, but ...
As a Canadian resident and taxpayer, I claim my partner as a dependant when I file my tax return. (He doesn't work these days, don't ask me why.) When he was employed, I was covered by his supplemental group insurance. (I'm self-employed so otherwise we pay for prescriptions, dental and eyeglasses and such out of pocket.) We are heterosexual and middle-aged, we have lived together some 12 years, we aren't married and we have not the slightest desire to be married.
Frankly, as a woman, I long felt betrayed by gay men and their militancy about marriage, which I have always regarded as an institution of patriarchy to be rejected, not embraced. But being committed as I am to equality rights, I absolutely supported their demands. Hell, I cancelled my burger order from the church group fundraising outside my grocery store, ravenous though I was, after I enquired whether the church would be performing same-sex marriages now that their legality was recognized and was told "no". ;)
Not everyone wishes to be married. We, for instance, are both atheists and therefore have none of the religious motivations that still drive many people to marry. And we are as perplexed by the institution (in the legal sense) of marriage as we are by religion.
Nonetheless we have accepted responsibilities to each other by cohabiting and those responsibilities are recognized by the state. (We could provide for those responsibilities in a private cohabitation agreement but have not bothered.) I am taxed as a person with an adult dependant, I will be eligible for survivor benefits under the public pension plan if he predeceases me, we have the same inheritance rights and support rights as if we were married, etc. (The only real distinction is that neither of us acquires ownership rights in our residence if one party owns it.) And no distinction is made between same-sex and opposite-sex couples, whether married or unmarried, in any of those regards.
We are treated as spouses by absolutely every public and private program that covers spouses. Just as gay and lesbian couples in our situation are, and have been for many years, including before same-sex marriage was recognized as legal here several years ago.
Why would the question of fraudulent "couples" be any different from any other kind of fraud perpetrated on benefits programs or in tax returns, for instance? Obviously, in order to qualify, people must make formal statements that they declare to be true, and making false statements in order to obtain a benefit would be a criminal offence. If they declare that they are a party to a domestic partnership in which they have financial responsibilities to the other partner, then surely that is no more dangerous a statement to rely on than a statement about anything else that might determine eligibility.
Of course same-sex marriages should be legally recognized.
But why non-marriage partnerships of either type should not generate the same benefits as marriage partnerships, and why progressive people would not call for that ... I don't get it.
For anyone interested, 2006 Canadian census data on the marital status of couples in Canada:
same-sex couples
http://www12.statcan.ca/census-recensement/2006/as-sa/97-553/p4-eng.cfm"The number of same-sex couples surged 32.6% between 2001 and 2006, five times the pace of opposite-sex couples (+5.9%). For the first time, the census counted same-sex married couples, reflecting the legalization of same-sex marriages for all of Canada as of July 2005. In total, the census enumerated 45,345 same-sex couples, of which 7,465, or 16.5%, were married couples."
http://www42.statcan.ca/smr08/smr08_118-eng.htm"Gay pride... by the numbers"
generally
http://www.statcan.gc.ca/daily-quotidien/070912/dq070912a-eng.htm"Consequently, married-couple families accounted for 68.6% of all census families in 2006, down from 70.5% five years earlier. The proportion of common-law-couple families rose from 13.8% to 15.5%, while the share of lone-parent families increased slightly from 15.7% to 15.9%. Two decades ago, common-law-couple families accounted for only 7.2% of all census families. Married-couple families represented 80.2%, and lone-parent families, 12.7%."
1 in 7 couples in Canada in 2006 had chosen not to marry -- and in Quebec it was nearly 1 in 2!