Does it ever get boring being married to the same person?

Ok look. Marriage is marrying someone that あなた deeply 愛 and never want to seperate. It doesn't get boring to stay with same person.
Unless if he/she is having an affair with someone else while they are married, that shows they are tired of being with the same person which then leads to Divorce.

あなた talking about marriage as if it's torture.
Those "40-50 years" should be happy and memorable. Therefore you're spending it with the person あなた love, then again that's why あなた married them.

*stares*
i'm 15.

To be honest, marriage is alot like life, it goes through cycles. Some days あなた can be very happy, and others あなた just go through the motions. Two keys to a good marriage are, picking the right person, and working on the relationship. To keep from being bored, あなた can find ways to keep the relationship fresh and growing. Have things あなた do together, but it's ok to do things that あなた enjoy that your spouse might not. Just be sure not to put allow yourself to be in a situation where あなた could develope inappropriate relationships. Your spouse should always come first. 愛 is not just warm fuzzies, it's a commitment. Feelings ebb and flow but commitment will keep あなた on track. With the right person and commitment あなた can make it through the up and downs of life. Marriage can be satisfying for a lifetime.

*Points at divorce chart*

It can be for some people. The definition of marriage is rarely recognized these days, yet so is love. :/

Ok. Yes- marriage is hard work, and a long-term commitment to another being, and OF COURSE you'll get on one another's nerves quite a few times, but あなた don't NEED to be with them until one of あなた die- get a divorce- but when you're considering marriage, あなた need it to be with someone あなた TRULY 愛 and won't get "bored" of. あなた may think this is cheesy- but when you've found The One, you'll know and that should be enough.

As for your friends, their teenagers, we like to experience new things and a lot of us are too immature to deal with a long term relationship (no offense to anyone, but its true), you've heard the term playing the field, right? That's what we usually do (not saying this applies to everyone).

If あなた think you're getting "bored" of the same person- take a vacation for a week または so. After all, absence makes the ハート, 心 grow fonder.