At what point can you no longer justify a relationship/ marriage? There is this saying that sometimes love is not enough, how many of you believe this?

For example, i have witnssed a couple risk there children lives by driving dangerously because they were argueing, the husband in question had cheated twice, once an emotional affair over the internet and the other emotional and physical. Lied a number of times, as did she telling him that they had money in the savings when in actual fact she spent it all. And a whole bunch of other incidences, but because she and he want the relationship to work they are willing to put up with anything.

I have stayed and gone back to someone, who lied to me, gave me thirty minutes of his time to break up with me, dumped me by giving me flowers, ignored me for weeks after i asked for his help , dumped me over the phone, while i tried to end my life he was having sex with someone else, and yet i still wanted and loved him, and that is just a small list of what he did to me! (the whole story is alot more complicated), and there is a amss of other incidences.

At what point do we draw the line and move on even though we still love the person?
For you guys out there what situation would make you leave even though you still loved the person, when is enough enough?

Why cant people realise that just because there is love there that the relationship is unhealthy?