Yes, social issues are of great importance in presidential elections since the resident has the power to impact these social issues. In fact, the social issues that matter to the individual should be used as the criteria by which the person decides how to vote in the election. People should vote for the presidential candidate whose social policies they agree with.
Presidential candidates are often judged by their stands on various social issues. Conservative candidates may gain a following by openly opposing certain social issues. By the same token, this can also work for liberal candidates. The stance on social issues is part of the whole package that each politician brings to the table.
There are definitely social issue voters on both the conservative and liberal sides. I have a friend who is so anti-abortion that she's willing to overlook Trump's bigotry, racism and sexism and vote on that one issue. There will be plenty of anti- same sex marriage folks who'll choose Trump. Hopefully there will also be plenty of marriage equality folks who'll support Hillary too.
Voters will be affected by the social views of their president- for example, Donald Trump's opposition of same-sex marriage could lead to issues for many voting Americans- Americans who may have their new marriages annulled by a Trump presidency, Americans who may be banned from visiting their spouses in the hospital, Americans who may no longer be allowed to adopt. These Americans will be voting when the election rolls around, and these Americans will take social issues into account when casting their votes.