When somebody crosses the line, anything that happens right after it doesn't matter. The opinion of others it doesn't matter, and trying to hide it is pointless... at that point everyone has to assume responsibility for what's been done and act accordingly.
So let's talk about love. One of the most common situations of crossing the line is when you, been in a relationship (married or just dating) falls in love for another person and actually admit to the other person that you love him (her). Maybe you didn't kiss or have sex, maybe it was just flirting... the fact here is that you crossed the line when you admitted to yourself that you where in love with somebody else. At that point you can repeat for yourself as many times as you want that nothing happen and that you didn't cheat... fact is, something happen and in your heart you did cheat.
the good news here is that if your partner didn't find out, life can continue as always and you can pretend, even to yourself, that things are the same. So you crossed the line and whether you saty in your relationship of your new love, it doesn't matter anymore!
Another situation is about respect. All couples eventually have an argument about something. Money, work, jealousy etc. Some people rise their voice very easily, some other ones are more thoughtful but everyone says things that regret later. The real problem comes in the first time that you say the F word to your partner. Arguing and sating harsh words is common, but the first time you say 'Fuck you" breaks the line of respect the very rarely is gain back again. After that thing can just escalate and get worst, the frequency will increase and unhappiness will settle in in the relationship.
In the same line with the last topic is sex. (This apply mostly to men). Experimenting sexually is inherit to any love relationship, but as humans we come from many different backgrounds and not everything is for everybody. There are conservative values and liberal values and many time they meet in a love relationship without any advise. Then it comes loving and pushing the bar. Man or woman sometimes don't expect things that seem normal to their partners and when is time to perform they might tingh of the other the worst. If a liberal woman in a relationship with a conservative guy try her best he is going to think of her as a whore, he is going to start questioning her background, how many sexual partners she has had and become jealous.
A liberal guy will try to experiment and lead things to something more kinky every time and eventually fuck her in the ass... nothing wrong there, she is going to freak out and refuse, or accept. she is going to question what he has done in the past with other woman but at the end of the day she'll ponder her love and his love and decide if she'll take it... no worries there, but the line has been crossed.
It is well known that man are more sexually driven than woman... Don get confused, woman are just as horny as man, but woman relate sex more with love than guys. Man can have sex anytime with any woman just like that. woman can have sex just the same but when in a relationship the mood has to be right.
Many times guys are drunk or high and they get horny. when the hit the bed all they want is to fuck... the mood is not right that time and they force their way and have sex with a partner that is very annoyed, maybe even upset... that it still ok... But when things are awry and he forces his way to sex pushing his weight (namely money, looks or brute force) then the line is crossed... she may stay for long time in that relationship, but her mind would be some where... probably thinking about how to find her way out, because she or he have crossed the line.
Then it comes violence. This is the worst of the breaking points. There is not much to say... maybe. There are many people that like it rough, but they are the exception. In most cases when one partner forces it way on sex the other is going to get hurt and the memory won't fade away... when that happens the relationship would be dead no matter what, they have crossed the line.
And this is the one I really want to talk about
The age difference. In a normal society it is normal to accept a difference in age. You can be call a sugar daddy or a cougar... same thing.
Normally people are willing to accept a fifty percent difference in age. and whenever you go above or below that difference then you know that you have crossed the line. From there it doesn't matter the difference... as they say, 'Same difference'. If you are twenty and you are dating someone that is forth two, you may as well date someone that is fifty. or if you are fifty and you are dating someone that is thirthy, you may as well date someone that is eighteen. My point is that once you cross the fifty percent difference, nothing makes a difference. Here the only thing that you have to face is the fact that seldom you''l be call a daughter or son of your partner. As long as you are unconscious of this it wont matter, you'll be happy for as long as you love your older partner. I have met many people;e that are attracted to older people... that is their thing and that is what makes the happy.So, in this case, they have crossed the line... but in crossing it they have found happiness! So crossing the line doe not matter anymore.