Being in an open relationship usually entails a dynamic of primary and secondary partners. A primary partner is almost like the single partner you would have in a monogamous relationship. Your primary partner is the person who you are emotionally and romantically invested in. In an open relationship, the agreement is simply that you can have sex with other people outside of the relationship. Such people are secondary partners.
Sex is an extraordinarily intimate act to perform with another person. Bonding hormones like oxytocin and a multitude of endorphins are released as a result of it. When it comes to sex, emotions are very high. It is extraordinarily difficult to detach your emotional life with your sexual life. There is an old saying when it comes to sex: someone will get attached.
Now, of course, this old saying is not true in all cases. According to the world-renowned clinical psychologist Dr. Chloe Carmichael who specializes in relationship psychology, in the case of one night stands, it is a very common occurrence that at least one person ends up developing feelings of attachment for the other person. Just given the level of intimacy and hormonal strength which sex produces, it is simply a challenge for many not to develop deeper feelings of romance as a result of sex. Firstly, if this happens to you, it is important not to panic or get down on yourself. This is a very normal thing. It says nothing about your moral character. Who you are as a person has not been redefined in a negative light. However, if you do not take the proper course of action, who you are as a person very well might be redefined. You need to understand that you are in an open relationship with a primary partner. The boundaries that you have set with your primary partner do not entail being romantically committed to secondary partners. Hence, you have a moral and deeply interpersonal obligation to ensure that you do not fall in love with your primary partner.
Right now, you might be at the stage where you’ve developed feelings for a secondary partner. This might eventually lead to you full-on falling in love with them. However, this is not a necessary result of such feelings. There are many ways you can seal the lid of such feelings.
The first is to avoid sharing deeply personal details with this secondary partner. When you are in love with someone, one of the biggest impulses that you have is to open yourself up to them. Sharing deeply personal details with someone is one of the first steps towards trust and intimacy in romance. Hence, to do this with a secondary partner would be a huge mistake.
Next, you really should avoid staying in constant contact with this secondary partner. You probably have the urge to text them throughout the day --maybe even all day, every day. This is an urge that you need to resist. The more you are in contact with this person, the more likely it is that you will end up falling in love with them, by furthering the feelings you have already developed for them.
Moreover, you need to avoid fantasizing about them. The more you play images of them in your mind over and over again, the deeper you are going to get in your emotional attachment to them.
Finally, you should probably express your feelings to this secondary partner. But, when you do so, you need to also bring up boundaries. Regardless of how you or the other person feels, you are in an open relationship that doesn't entail secondary romantic commitments. If you are going to continue to have sex with this secondary partner, appropriate boundaries in light of your open relationship need to be established explicitly.
Now, all of this advice might work for you and you might be able to continue having sex with this person with no strings attached. But, this is not a guarantee. You may have implemented all of the above advice, but still, you find yourself falling in love with your secondary partner. In this case, it is probably best that you cut ties with them. It is unfair to all involved and fundamentally, if you act upon such love, you have committed a negative moral transgression that will end up hurting a lot of people.