There is a basic distinction worth mentioning, basically because some (like GOP Sen. Rick Santorum) stated that the Lawrence decision implied gay marriage. I still don't think it did, even though the SJC obviously disgrees. For one thing, Lawrence overturned a law, an established and long-accepted role of our higher courts. For another, it involved the state prosecuting private consensual behavior. Gay marriage, on the other hand, is not about lifting the criminalization of social behavior. Rather, it concerns citizens appealing to the government for a certain kind of recognition of their intimate relationships. To put it starkly, Lawrence involved removing the government from the bedroom; gay marriage proponents would invite it back in, ostensibly in an affirmative way.
This is a distinction worth making, I think, since the rhetoric has gotten a bit heated of late. The talk of discrimination is particularly worth unpacking. Discrimination means several things -- all of them bad thing, to a modern liberal. Is it discriminatory that only a natural born U.S. citizen (age 35 and over) can be president? Sure. Does that necessarily mean we should amend that part of the Constitution? I don't think so. Under current law, a 14-year-old Canadian girl cannot be president. It worth drawing the line somewhere, and that girl would fall on the side of the line that has no eligibility now (because of age) or ever (by dint of birthplace) to become president. I think it's worth elucidating that some discrimination can be inoffensive, even salutory. I don't know whether the current definition of marriage is either of those; the consequences, intended and otherwise, of huge social changes can be troublesome to foresee.
In spite of that, I don't see the institution of marriage coming apart if gays are allowed to join the club. I think any discriminatory conflicts could be as easily settled by civil union, or by simply not allowing marital status to intrude into policy. But I can't see calling marriage a fundamental right (see above). The right is to be treated equally, under the law. I don't see how that's possible when the law affords certain privileges to married couples and makes no accomodation for couples in the very same circumstance save the access to government-approved marital status. Thus, I'm well aware that something has to give in this situation. Amending the Constitution seems like the worst way to go.