Do you prefer the idea of being in a relationship in which you and the person you love both invest a lot, or are you the kind of person who prefers the idea of being free and not invested in a relationship?
To “invest” in this context is not about (or not much about) spending money. It refers to putting more of yourself into the relationship, and making choices on the basis that the relationship is going to continue, rather than assuming that the relationship will end and making choices on that basis.
If you are in a relationship but believe that your relationship is going to end, or is not serious, you will naturally invest less. You will naturally care less, think about the other person less, make few or no future-related decisions and you may be looking around hoping to meet someone else.
If you are the kind of person for whom the idea of investing in a relationship sounds like locking yourself in a prison, you will avoid getting into a relationship at all, preferring “freedom”.
Some like the idea of “freedom” but also want to be with someone who invests in a relationship with them. The trouble is that this is an unstable arrangement because most people are not happy to be invested in a relationship with someone who is not similarly invested. So if you find yourself more or less heavily invested in a relationship than is the other person, it might be worth considering making some changes to create a better balance.
For most, being “free” conflicts with being invested, and it is necessary to choose whether you want a more invested relationship or a less invested one. The problem many have is that they fall between the two stools of invested or “free” and that ends up destroying their marriage.
Marriage tends to mean more investment than unmarried relationships and dating relationships. And at first sight, it might seem obvious that a monogamous relationship is a more heavily invested one than a poly one, but that is not necessarily true. Some in poly relationships are very heavily invested while some in “monogamous” relationships are not very invested.
For example, imagine a poly relationship between three persons who live in a very conservative small town. Suppose they have an agreement not to engage sexually with anyone else, and not even to put themsleves in situations that might lead to them wanting to engage sexually with anyone else. Further imagine that they have a rich shared fantasy life, and that they have bought a house together. And suppose they have bravely come out publicly as being in a poly relationship, and are now shunned and alienated by everyone in their town. Even if they break up, they are still not going to be accepted by the people in their town. All in all, this might well be quite a heavily invested relationship.
By contrast, imagine a “monogamous” marriage in which the parties don't live together, are often apart for long stretches and don't speak on the phone regularly when they are apart, don't think about one another much when they are not together, and either engage sexually with others or at least have rich and complex sexual fantasies that are rarely anything to do with their spouse. It is at least coneivable that this might be a less-heavily invested relationship than the poly one described above.
One of the decisions that tends to lead to greater investment is the decision to seek a sexually exclusive marriage. For most rational and monogamously-inclined individuals, the thought that sexual exclusivity will not be possible with a particular person discourages them from investing much in the relationship.
Some prefer not to take the risk of investing heavily in a relationship. Perhaps they have been badly hurt in the past, or perhaps they have been in a stale and boring marriage from which it was difficult to get out.
But if two individuals who each independently want to be in a sexually exclusive marriage meet, and they get together, then, assuming that they find a way to have a good relationship, they may well both invest heavily in the relationship, each making decisions for his or her own life on the assumption that the relationship will be permanent. Every decision you make on that basis is a piece of investment in the relationship, and the more you both invest, the deeper your relationship can become, but the worse it will be if the relationship fails.