When Kerry first pushed for the deadline (actually 2 deadlines) on April 1, 2006 in the NYT op-ed, this was a minority position. It was not even close to 50% in any poll that offered several alternatives. This is a case where Kerry actually did lead public opinion and withstood a huge amount of flack to do so. (Remember that Obama and Clinton didn't go to this position until about 9 months later.)
On investigation of torture, Kerry has gone beyond Obama. He did follow Obama on not prosecuting agents, though he narrowly confined that to those that stayed completely within the legal framework they were given. I know that you are right that this is NOT the standard we imposed on Germans. There, I suspect it is empathy - even if misplaced - for the lower level people, who trusted their government and did things they now have to live with. I don't see this as liberal or conservative.
On gay marriage, Kerry has moved a considerable distance. He is not a leader on this - and I would really not expect him to be one. This is an issue where there has been rapid change in the last 40 years. In the 1960s, just coming out of the closet was a big deal. These changes are wonderful and needed. The biggest determinant of people's position on this is not political leaning, but age. People coming of age now have grown up with friends who were openly gay and often know older relatives or family friends where that was an unquestioned given. This was not true for those of us coming of age in the 1960s - and the late 60s were extremely different than the early 60s.
Where Kerry was a leader on gay issues, it was in the 1980s where he was an early advocate for equal rights. He still is motivated to take on some advocacy for that reason. In addition, his position of civil unions with FULL federal rights in 2004 was more than any previous nominee had as a position. He was clearer and stronger on that than the significantly younger, non-Catholic Obama, whose church allows gay marriage. There was some risk in 2004 in the position Kerry took. By 2008, there was none in Obama taking the same position as the country had moved on this.
A 2004 Online News Hour (Jim Lehrer) had this:
In the Senate, Kerry has supported the expansion of rights for gay couples. He cosponsored the Permanent Partners Immigration Act of 2003 that would allow gays from another country who are in a committed relationship with a U.S. citizen to immigrate to the United States under the same conditions as someone married to an American. He also cosponsored a 2003 bill that would entitle domestic partners of federal employees to the benefits available to those who are married to federal employees.
He also, per Bay Window, lobbied behind the scenes to get a needed vote to prevent a referendum that could have ended the right to gay marriage in MA. The source was the state Senator who attributed his change in position to a call from Kerry. It would be good if strongly stated words accompanied the action - but this is a case where actions speak louder than words. In 2008 - in response to Blue Mass's Cambridge Paul he said that he was not against MA's gay marriage - though it was convoluted. In terms of MA, he is nowhere near leading on this, but at the national level - and the Senate is a national body - he is one of the leaders.
When we all went to Take back America, the best session was with Roy Wilkins, Jesse Jackson and a biographer of MLK. One take away was that African American civil rights needed both the energy of the protest movement and the quieter inside government writing of law that would make the changes. Without the energy and vision of the activists making equal rights an issue. I think it is completely normal that it was the activists in MA and elsewhere that first pushed this change. It is very unusual when a politician does. In the 1960s, JFK and LBJ were way behind the activists - yet they deserve credit for moving when they did.