Not sure what your company policies on travel are, but one of the reasons I continue to travel for work 100% of the time is that I am able to "in lieu" almost anywhere as long as the cost is reasonable. At the end of the day, my company doesn't care if I go home to my apartment in New York or to visit a friend in Chicago. It is also nice because my family is from Ohio, so I am able to visit them more often than I would if I didn't travel for work.
Also, I am able to "in lieu" a spouse/significant other/friend and as long as the costs are in line with what it would cost to fly me home, I am able to fly a friend out to me. I think this is a great deal especially if you have a significant other, you get to see each other and it is like a little vacation that the company partially picked up.
The biggest reason I travel is the points/miles. I just booked my first award trip to Asia in first class with miles. There is no way I could afford something like that out of pocket.
As a side, I am a single guy with no kids so other than the ridiculous amount of rent I pay, there is no reason for me to be at my apartment.
As for the GF situation, like I said I am single so probably not the best to comment, but I think if she is fine with your travel you both can really benefit from the points/miles you'll accumulate and if you can fly her out over the weekends. Personally I think it'd be nice to have a break from each other during the week and then the time you spend over the weekends is just that more special, but like I said before I am probably not the best person to offer advice in this area.
I think in your situation one of the biggest down sides is that you have a mortgage. Most of my co workers that travel and have their own home seem to complain most about that.