Yes, this is very normal to see an object like a portal while performing astral projection.
While performing astral projection we basically see what we have in our minds and subconscious. If you saw portals while performing astral projecting, it simply means that it was what you were having in your mind / subconscious and because of this it appeared in the astral realm while you were travelling.
The astral environment is thought and feeling sensitive, so if you feel afraid you will attract fearful experiences, if you feel positive you will attract positive experiences. Your thoughts will present themselves before your eyes so it's wise to pay attention to what you have in your mind and not let your thoughts run wild. If you are attempting astral projection be aware of your state of mind, it should not be practised if you are depressed or frightened of what you may find. Like attracts like in the astral.
As I said, it works with thoughts, positive and negative energy, etc. Same thing happens with many other things such as: fear, bad thoughts and negative energy. We mostly see in the astral realm things we are afraid of and attract negative entities because we think about them, and in order to make the dissapear we simply stop thinking about them while astral travelling, and by this we stop attracting negative energies and thoughts in the astral plane so they dissapear from our sight.
For example, if i perform astral projection while i am full of negative energies - i might attract negative entities and sights in the astral realms. For another example, if I think about my fears and about my problems, I will see them in the astral realm. So you probably thought about something associated with portals, which is very " normal " and logical for something that you might see in the astral realm.
You might also see in the astral many things that might be questionable, so do not get afraid since this is totally a natural thing that happens in the astral realms. There is nothing to be afraid of.
In your case , you were probably thinking about portals, probably because you imagined that it would be the way the astral plane would look like. We basically get to see what we expected to see, thus we build our own realm when it comes to astral projection. Many people experience and see different things while doing this. That is simply how it works.
This all can be defined as " the law of attraction " which is always associated with astral projecting and any forms of other dreaming in general. To sum it up, the law of attraction says that by focusing on specific thoughts, we can bring results. In the astral it can be positive, negative, or just something that you're thinking about.