How do I setup a time for romance in a campaign based on fights and quests?
Romance typically doesn't happen in the middle of an epic battle. It happens around the campfire later that night, or in the local tavern. The best way to include such subplots is to actually role-play the interactions between interested parties when the group makes camp for the night.
If both halves of a romantic pairing are active party members, maybe they can make passing comments or quips while the group is fighting/exploring/whatever, but these interactions should be kept quick and limited in number, since there are, in fact, more important things at hand. In the real world, a romantic couple is not constantly remarking on their partner's "sweet tush" or making double-entendres about what might happen later that night, especially not when they're in the middle of something more important.
You can (and should) also cut away from the lovebirds to ask what the other party members are doing during this time. This gives them an opportunity to remain engaged in the game and, not incidentally, an opportunity to develop their own non-romantic subplots. This will enrich the game as a whole and hopefully keep everyone entertained.
How can my players win the love of their target ? (Oh my god it sounds creepy)
I would do this entirely through role-play. No dice. Using game mechanics to describe romance (and worse, sex) invariably objectifies the entire process. It reduces the interaction to numbers and obliterates the magic that is Love. If players want to pursue romantic subplots, they do it by role-playing their interactions with their love interests.
Also, try to keep things as mature as possible. Don't let it degenerate into a series of crude (out of character) jokes from the peanut gallery.
Maybe use the occasional Charisma check for important turning points (ie: the player used a REALLY cheesy pick up line... Did she laugh, or did she brush him off?) But do this SPARINGLY. The majority of such things should unfold naturally through role-play.
How can I entertain players that are not interested in such matters.
By keeping these romantic subplots short and to the point. A real relationship doesn't develop all at once. It happens over time, after several meetings and time spent talking with one another. Play these one scene at a time. Ideally one scene per game session, two tops. Stop at a good, heart-warming moment. Move on to other, non-romance related things. And try to make sure the majority of the game revolves around traditional adventure - which all players will presumably enjoy.
ABOVE ALL, DO NOT INCLUDE SEX "ON SCREEN." I mean don't include it at all. When characters start kissing, you fade to black or change the scene to something else. This isn't about being a prude - playing out a scene where two characters are having intercourse takes the game into an entirely different arena. It's going to awkward for everyone, and will almost certainly devolve the entire subplot into dick jokes and misogyny. There's probably going to be some of that no matter what you do, but you want to avoid it as much as possible, as it will only cheapen the whole experience.
Female characters should not become pregnant unless all players (ALL players) are willing to use it as a plot hook. Dealing with a pregnancy (let alone a baby) is going to have a significant impact on the entire game - not just the characters who are the prospective parents, but everyone around them as well. Yes, unexpected pregnancies can happen in real life, but in an RPG campaign it's a HUGE wrinkle that's likely to derail everything else. This will only end in tears.
