As long as you don't plan on starting anything serious with the new person, then I would say yes it is still okay to like a past flame.

You never forget your first love; if you say you did, you lie,
If you say you did not, Better try.

Your past relationships make you who you are; whether they were good or bad, you grow from them one way or another, either for good or bad. Depending on what you mean by "like" your past loves, you can either move on with your new "love" or go back to your old one (with varied results). Are you asking can you love several people at once with the same intensity? I personally don't think so.