Edit the raws so that orientation is different if it's a problem. Hell even then you might still end up with straight dwarves who
a) are not well split in gender
b) none of them want to get married and have kids, but are fine to take lovers
c) are not the right corresponding ages (this could be remedied by making sure the history is a custom set number, say around 20-70)
Honestly you're better off modding because that'll be a less offensive, and more intelligent method.
In general with only 7 dwarves you're screwed due to statistics. 20% of a population, but each of these dwarves are generated if I remember right, giving them the same odds to be what is possible.
Just like even if you play the lottery millions of times, you may never, ever win. You might end up with all female dwarves who will only ever take lovers.
Honeymoon suites, modding, and accepting a migrant wave or two are your best bet
You'll still have to murder the dwarves who won't get married and have babies, but are straight.
EDIT: Also here is a non-modded solution
1 ) Make as many slabs as there are dwarves in your fort.
2 ) Make a tunnel to the magma sea.
3 ) Make a retracting bridge over it
4 ) Format a military squad (Let's name it No Babies), civilian clothing only
5 ) Now establish which dwarves are gay, which are bisexual, and which are straight
6 ) establish which of the straight and which of the bisexual will get married to the opposite gender
7 ) establish which of those who will get straight married will have babies
8 ) Put as many dwarves as necessary into the military squad
9 ) station them over the bridge
10 ) pull the lever (kronk)
11 ) engrave the slabs of the newly deceased
12 ) repeat as many times necessary
13 ) enjoy the handful of dwarves who will make babies
Don't forget to ensure no dwarf sees the deaths, put 2 locked doors on the way to the room, and unlock/lock as necessary