After our son was born, if my husband had said to me that I was now in second place and all his attentions would from now on be concentrated on our son, I would have been devastated. I think such a statement would be likely to damage most relationships. A happy and relaxed relationship between parents is, I think, not only better for them but more likely to create a secure and happy base for the children.
When I was very young, my mum was quite possessive and over-protective of me, and dad was, to some degree, kept on the sidelines. With hindsight I realise that it fulfilled her need to be centre of attention but it affected my confidence and ability to be decisive and independent. It also meant that, at a young age, I did not have a very affectionate relationship with my dad, although it improved as I got older.
Of course, when you have children your priorities change, and going out or going away requires careful planning and organisation. But I think it's good for children to spend time away from their parents and with other relatives - as long as they're happy to do so. I was very close to my granddad and used to stay with him (my granny died quite young) now and again. It's a relationship that was a great source of comfort - and fun - to me. Parents are probably the most important people in young children's lives but that doesn't mean that they don't need or benefit from other close relationships. Our grandchildren often stay at our house and have a lovely time. We tend to have more time and energy to play with them - and it gives their parents a bit of a breather.
Having said that, I do think that the needs and feelings of children should always come first when marriages/relationships break down, and that might mean parents having to pay extra attention to their children and to keep any new relationships very low key for a fair period of time.