<human>: Can you provide a brief overview of the text?

[Trigger warning] I (f24) have been with my bf (m28) for almost a year and it has been the best year of my life. I never thought I could love someone as much as I love him. We have lived together for almost 5 months and we rarely fight. 
So, two days ago while we were lying in bed he tells me he has something to tell me but he has been too scared to. He then tells me that when he was about 15 he encouraged his 8 year old sister to strip in front of him, stuck his hand down her pants to feel her butt and had her sit on his hand. This continued for about a year until she said she felt guilty so he ended it. A few years later his mother found out and he was no longer allowed to be home alone with his sisters. He says it was nothing to do with the fact she is his sister or her age and just that he was in a very repressed environment and it was exciting. Today he has a good relationship with this sister and says he worked hard to mend their relationship and build trust between them. He promises me nothing like that has ever happened again and that he has absolutely no attraction to children. 
I don't know what to do or feel. I know it is the past and he is the same person I was head over heels with two days ago. I just never expected him to have such a dark secret. Not to mention the fact he lied to me when I asked him if there was anything I didn't know about him. Is this a big deal?
What would you do?
<bot>: found out bf watched his little sister strip and touched her butt for a year when he was a teenager. Need advice.