I just want to build up on what Woodrow said -- if the cap'n doesn't mind ;)
As Muslims, we don't live for this life, but the hereafter. We look at this life as nothing more than a quick bridge to the place that we will eternally reside in. It's how we act on this bridge that determines how our life after that will be, so if we do good deeds in this life and we do them not for some worldly cause like prestige, graces of women, or just getting more money -- but instead doing them for Allah's sake while at the same time worshipping Allah the way He deserves to be worshipped, then we have a much better chance of being rewarded and living happily in the hereafter. In that case, marriage in Islam is not necessarily for "eternal love" or whatever, but for having a partner in life who can support you, care for you, fulfil your desires, and through their words, actions, and their love, bring you closer to Allah and keep you on the right path to entering paradise.
Maybe you shouldn't be so quick to rule out becoming a Muslim -- frankly what have you got to lose? Perhaps the reason why you don't align yourself with any religion, and especially Islam, is because you don't know much about Islam, and contrary to what some people said, I think that hanging out with us here on the forum could give you a lot more insight into what Islam is and, maybe more importantly for you, what it's like to be around Muslims. So I say keep that possibility open, and you'll see we're a nice bunch :)
As for what you said...
But I don't believe any religion can have the ultimate truth. Islam says that Muhammad is the last prophet. But surely God can change his mind
God CAN change His mind, He can do anything, but thinking about it logically, why would someone who knows EVERYTHING, including the infinite future and past of the universe and each of us, ever change their mind? Either way that's not the point. Mohammed (pbuh) was sent to mankind as a last warning after innumerable other prophets (pbut) came before him to warn mankind about their evil actions and their betrayal of their creator...and to give them a chance to repent and seek His mercy. What we do with that warning is completely up to us, but we are made aware of the rewards and consequences of the paths that we choose, and are also provided with the ideal path to follow in order to be in Allah's grace.
Take care man, and welcome to the forum :)