Here's my take on the subject.
Everyone's been taking the "Boba's a deadbeat" approach or the "protect his family from enemies" approach, but it could actually be more tragic and sadder than that.
What if Boba didn't know he fathered a daughter?
Hear me out. Boba is a bit older, wiser. He's had a few close calls where his life flashed before his eyes, as in almost died from a hunting job. While not necessarily looking to have a legacy, he's probably rethinking his view on the "no sex" thing, after all he could die a violent death without experiencing it, let's face it, everyone grows up and interests change.
Boba's living fast and he kinda likes this other bounty hunter chick, cause she's strong and again let's face it, there's not many attractive women in the bounty hunter gig. One night they're going for the same bounty and end up on the wrong end of an ambush, they get out of it, bag the bounty, but barely. Both still under the rush of the fight and scared to an inch of their lives, and fearing that they wouldn't live past tomorrow,well.... they end up doing it, in the heat of the moment.
Boba, not exposed to the mysteries of life goes "it was great, cool, we should do it again some time" and then heads of to hunt other bounties, forgetting about the chick completely.
Sintas, mistaking Boba's inexperience for being a deadbeat, decides to have the kid and tells her that "daddy left us when you were still a baby".
Baby is mad, cause mommy is the only person she has (and who else would she believe?), so she vows to kill the deadbeat daddy when she's all grown up.
Meanwhile Boba's gotten even more mature, even wiser and he's thinking he should now have a legacy.
The irony is, his legacy wants him dead.
The only glitch in this theory is the holovid pic of a mando and Sintas holding Ailyn, but then again Sintas could've just posed with Jodo Kast or some other wayward mando.