If the NRA announced they were for abortion and gay marriage, they would lose a lot of members
You are making stuff up.
NRLC has NOT taken a position on the Gay Marriage issue.
The gay "marriage" issue is a tougher sell. What real direct harm is it doing to anyone? Especially anyone that doesn't believe in a spiritual world or God? The common counterargument is "Well if you don't think gay marriage is right then don't get married to someone of the same sex!" The average person couldn't give a damn about the "overturning of a centuries old definition of marriage". That is merely a concept to them, and concepts don't get them excited.
But if real people are being really and truly hurt then the case can be made to stop that from happening.
So I think it is important from a strategic point of view that organizations that support right to life issues specifically make no comments one way or the other on these other issues. And even if NRTL does support Rob Portman doesn't mean they support him on everything. How many of us are big Ronald Reagan fans? How many of us agree with his 1986 illegal alien amnesty bill? I am not a supporter of illegal alien amnesty just because I am in general a big fan of Reagan and his presidency. NRTL is not a supporter of gay marriage if they happen to support Rob Portman. If they have gone on record explicitly in support of gay marriage, then I believe they are wrong to do so. It doesn't advance the cause against abortion, and it just muddies the waters in general.
And as far as the "gay marriage" issue being an RTL issue, well here are some others if you're willing to stretch things a bit:
1. 2nd Amendment: If I can't own a gun, I can't protect my right to life when accosted by a criminal
2. Property Rights: If my property rights are infringed then I won't be able to establish a place to make my last stand against a tyrannical regime and/or common criminals.
3. Secondhand Smoke: If we don't stop all smokers from smoking right now and forever more, then I might accidentally inhale some secondhand smoke which might reduce my lifespan by several milliseconds. Horrors!
4. Medicare/Medicaid/Obamacare: If I don't have free and easy access to the medical care I need then I could die prematurely.
etc.
And if they announced they were AGAINST abortion and gay marriage, they would ALSO lose a lot of members. So, wisely, they stick with their mission and stay out of non-2nd-Amendment-related issues.