An Idol
Chibi Usa/Rini idolizes Mamochan (in both manga and anime) as the perfect man, and doesn't seem to realize or focus on the fact that he will one day be her father. All she sees is a guy who takes care of her, who loves her, who's with a ditz named Usagi, and who she wishes was instead with her. She doesn't think 'wait a sec, he's my dad, I can't do that'. She just doesn't think of it that way and so proceeds to fall in love with her dad's past self.

Wanting to be an Adult
Chibi Usa's main problem is that she is chibi/small. She wants to be a beautiful adult like her mother (whom she also idolizes) and in the past, she also looks upto Usagi because Usagi is a grown woman who can do many things (one of them is having a relationship with Mamochan)

Since in the past Chibi Usa and Usagi are more like sisters than mother and daughter, it is easy to see how Chibi Usa will think she's better for Mamochan than her lousy numskull sister-like figure Usagi. All of this is tied in with her desire to be adult, to be bigger and more of a woman. To no longer be a child. She wants to play the part of a grown up, to fall in love, and the only man in her life to look upto and fall in love with is Mamochan.

Chases After Mamoru - NOT After Her Father
She doesn't focus on the fact that he is her father - who knows why - maybe it was just something Naoko forgot to expand on - but she also doesn't say 'he's my father, oooh yah I want him bad', she doesn't say that ever. She sees Mamochan as Mamochan. That's all. Boyfriend of Usagi and a guy who gives her (Chibi Usa) special love and attention she's never had from any guy save her own father.

Understanding of Loneliness: A Common Bond
It's understandable that a lil kid will idolize and become "in love" with an older man who gives her love and attention especially when the girl feels so alone - as she first did in the past - away from her family and friends. Mamochan seemed to be the only one who understood her feeling of loneliness, of being orphaned (remember, he was an orphan as well).

A little girl who wishes to be adult, no longer chibi (she has been chibi for 901 years), who is placed in the past without her parents and the friends she knows and loves, this little girl proceeds to fall in love with the first guy who gives her love, understanding, and attention. She doesn't fully understand that he is her father. But she doesn't chase after him BECAUSE he is her father, she chases after him because she genuinely needs his love and affection.

Falling In Love with Elios Changes Everything
All this changes though as she falls in love with Elios/Pegasus. She now has a man in her life she also idolizes and loves, and who loves her back and IN THAT WAY and who she can be with as an equal. Read more about Elios. Elios is the one who makes her feel like a woman, not like a chibi. Elios shows her that she is a beautiful maiden, and one who has won his heart. This is something she never wished for from Mamoru and never would have gotten anyway.

Running Away From Hostility To Someone Who Will Understand
Her actions are forgiveable. She may be annoying but she, like her mother, has a heart of gold, she is loyal and loving, and will give up her life for the ones she loves. She and Usagi have many tender moments, and one of the things I love best about Chibi Usa is that when Usagi is sad and upset, Chibi Usa will try to cheer her up. Ex: manga volume 4 (5?) Usagi is crying and upset after losing her friends to the Black Moon family, Chibi Usa comes into Usagi's room to comfort her, and her little cat ball, Luna P says 'Don't cry lady'. Usagi yells at Chibi Usa and Chibi Usa runs away crying because she only wanted to make Usagi feel better. Usagi realizes this a second later and feels bad for yelling at the little girl.

Facing such hostility from the one person (usagi) that Chibi Usa hopes to be friends with, it is foreseeable that she will run to the only other person who gives her love and attention - Mamochan. In her heart, Chibi Usa knows mamochan and usagi belong together. But in her childish mind and confused state of emotions, all she fully understands is her need for love, care and friendship. She finds this in Mamochan.

Conclusion
Chibi Usa, in conclusion, is not some incestuous, horny little girl. She sees Mamochan as her friend, needs his love and attention and in her wish to be grown up, wants to know what it like to have such an ideal man as her boyfriend. She chases Mamochan for who he is, as Mamochan, and never chases him as her father.


Below is something my friend ChibiKris wrote about Chibi Usa and Mamoru:

Child Psychology: It's a Little Girl's Nature
Chibi Usa is not in LOVE with Mamoru. She has a slight crush if you can even call it that. He's the only male role model in her life during this time. She looks up to him. Remember at first she doesn' t now that he is her father and Usagi is her mother. Even when she found out, to a young girl that wouldn't seem real. She is also jealous of Usagi (much like a sister.) She wants what Usagi has and that means she wants Mamoru to pay attention to her...it purely the selfishness of a little girl.

Study some psychology and you will find out that she is quite normal. Little girls at her age worship their fathers and idolize other males like big brothers, sister's boyfriend's etc. That whole bit about being grown up in SuperS....what little girl in her right mind wouldn't die to be an adult. You get to do all the "fun" things. She's not warped or sick because she looks up to Mamoru. She is not thinking of him in any sexual way. Don't try to replace a childish crush of infatuation with a sexual crush of an adult.