Been thinking about this middle of the night (my brain tends to wander more than usual when I'm not feeling my best) and this relationship is not discussed very often, but it's important when thinking about Harry's charater, she was a huge part of his life for a long time.
Ok a lot of people (yes, I am talking about you Harry/Selena fans) say that he wasn't in love with Beth any more, and he would have left her for Selena.
Looking at Flight and how Beth and Harry's relationship is, that is definately a couple very much in love, even after 16+ years. There is only 6-7 episode between Flight and when Beth dies in Keep It In The Family, average Casualty time span between is a week, so in the Casulty time line a maximum of 7 weeks had past. I don't believe Harry could have fallen out of love with Beth in that amount of time, and feel their problems lie elsewhere.
We know they had a fairly active 'bedroom life' with 5 children between 18 months to 16 years when we first me the Harper brood and that they don't often get to spend time together with just each other
We know they both have very demanding and time consuming jobs, we also know that Beth had a few (probably well founded) suspecisions between Harry and the Au Pair's. Daisy their youngest is also at a very demanding age (Harry mentioned that he was up with her all night because she was teething) and that Beth and Harry didn't spend as much time together as they should.
So I feel that Harry (being you typical man, sorry but he is) if he's feeling neglected/lack of attention tends to follow his 'other' brain. You can see this with his realtionship with Anna, that he loves women in general (he flirts with most women) and has a hard time saying no when something is on offer, remember Anna pulled away first, and Roxy's comment when he left for dinner with Selena - She's not the first and she won't be the last.
So in my opinion if Beth didn't die, he would still be happily married to her and the whole affair with Selena probably would have been only a one night stand (along with several more in between.) I think it probably would have got to a point where their children were old enough to look after themselves and they would of had more time to spend with each other, and their relationship would have grown a lot stonger.
So there it is my opinion of Harry and Beth, this doesn't mean I'm no longer a Harry/Maggie fan, a couple very much in love but who due to work and children as higher priorties they just seemed to not appreciate each other as much as they should. And don't forget how often Harry looks at Beth's picture which still sits close to or on his desk in his office.