You are probably feeling very confused and vunerable at the moment. Love can blind even the most sensible of people.
I can only give you my opinion on this matter, as you will find, there are as many opinions as posters here on GC.
As to your question "can men be attracted to both girls and women?" the answer, if GC is any barometer, is most definitely yes.
The attraction base on GC is as varied as the opinions. I think the majority are exclusively attracted to pre-pubescent children, but there are many who like all ages, just teenagers (hebephiles) and a few who are bi-sexual.
Even though I have an age of attraction of 3-9, this has been a gradual realisation and it did not prevent me from marrying and raising a family. Because I was exclusively attracted to children, when my wife and I made love I usually fantasized about girls - it could be that my wife fantasized about firemen! She never told me.
Now, down to the difficult part. Firstly, I think your parnter betrayed both your trust and your daughters. I personally am vehemently against incest, and my abhorrence of it also includes any relations between a man and a step-daughter.
I just think that a mans role in that situation is to be a father, and nothing more. In my opinion your partner is not a pedosexual, he is a child-molester.
If you stick by this man you are storing up trouble for the future. Love is blind, and you are in danger of walking into a dangerous relationship with your eyes closed.

Has it occured to you that the sole reason he started dating you was to get close to your daughter. That may be a blow to your ego, but his motivations must be in question. Would he have hit on you if you had been childless?