If two people who love each other wish to spend the rest of their lives together, and wish to solemnise this in front of their friends and family, I have no problem what so ever - regardless of gender combination. How does another person's marriage affect mine or anyone else's besides those in the marriage in question? It doesn't. If it does - there is something seriously wrong with the person who is affected (yet not part of the SS marriage) marriage.
Mostly it is about love, and respect. But it's also about equality. In many countries, those in non-recognised relationships are denied the same rights as their heterosexual counterparts - despite paying the same taxes, following the same laws, working as hard as everyone - they are denied the same rights.
No one is asking to take rights away from straight marriages - all they are asking is to be given the same rights, be recognised as equal to their fellow human.
There are also health benefits - for the couple, for the family, for the children. Children who grow up in a household where their parents are considered to be 'less equal' than others are also affected. Health insurance benefits are difficult to extend to non-bio children if you can't marry your other half, nor adopt their bio child as your own. Children who have parents who's marriages are recognised as equal to others (Same Sex and opposite sex) fare better in health than those who are sidelined by society merely because of the gender of their parents.
I feel that my marriage is as equal to my friends and families marriages - be that they be straight, or not. Thankfully, in my country, it is legally recognised as equal also. Unfortunately, it is not so for so many people. For some, just being gay can be a cause for the death penalty....how is that right?