Hey SaBo, no, it is not selfish. Everybody has feelings.
The other night my mono boyfriend and I had this same talk, in which I told him of my feelings and how somehow even though I'm poly and don't think I can change, it still freaks me out the idea of him dating anyone else at the same time as he dates me. I told him I know this is hypocritical and that I wasn't asking him to not date other people but that I really did want to talk about it. He said that he doesn't view it as hypocritical. He is not interested in dating anyone else. His point (and mine) is that everybody is wired differently.
I really liked what New had to say. I too can get anxious and need constant reassurance (I have an anxiety disorder according to my shrink and should really get on meds soon), but sometimes people do not need the same things. My BF does not feel the need to date more than one person (and he reckons that might be a bit of a challenge as well in terms of time and being there emotionally) and yet I do. I keep asking if it does not bother him me being poly and he says all he really wants is for me to be happy and to be at peace with my decisions.
In terms of a relationship and both partners feeling secure often the two partners can't be treated exactly the same. It is similar to our needs in society. Some people need antidepressants to function. Some people need more sleep than others. Some people can't stand being alone while others really need their alone-time. We can't treat everyone the same and everyone has different needs in order to feel secure. So long as we can ensure that both sides are happy and feel that their needs are met and that reasonable compromises can be reached this is fair. Your partner may have no issue with you being emotionally attached to someone but may place restrictions on physical aspects, whereas you may find the emotional attachments more of a threat.
In discussing this with your partner(s) just make sure that it is clear that these are your feelings and not necessarily demands you have on your partner, and make known your own needs (eg to spend a certain amount of time with your partner, to feel fairly in control of certain aspects of their interaction or to be informed, etc.)
Everybody has different needs, and it is well possible that you may need someone to be the exact opposite of you in terms of needs (in my case mono and very secure in himself) in order to feel secure yourself within a relationship.