Has anyone been involved in a semi-?

By "semi-", I mean you have done pleasurable acts with a couple, however have not had full with both partners. I guess you can say I am the outsider (the 3rd person) in this group. I just want to know what kind of feelings do you experience, do you get attach to the couple, or is it a type of relationship with "no strings attached". I know I should have that mentality where I should not have any feelings for them, but idk...i find myself that I dont have feelings for both of them, but I like and enjoy hanging out with them alot! Mayb its the chase that I get from him, he flirts with me and makes me feel different. What Im asking is am I the only one feeling like this??? HELP, thank you