“The burden of proof for setting aside a domestic contract is on the person alleging that the domestic contract is invalid. The party seeking to escape the effect of the agreement has the onus to show that there are grounds for setting it aside (see Dougherty v. Dougherty, 2008 ONCA 302, [2008] O.J. No. 1502 (Ont. C.A.), at para. 11, Loy v. Loy, [2007] O.J. No. 4274 (Ont. S.C.J.), at para. 161), Shair v. Shair, [2015] O.J. No. 4883 (Ont. S.C.J.) at para. 44).
As a general rule, courts will uphold the terms of a valid enforceable domestic contract. It is desirable that the parties should settle their own affairs if possible, as they are more likely to accept their own solution to their problem than one imposed on them (Farquar v. Farquar, [1983] O.J. No. 3185 (Ont. C.A.), at paras. 19-20, Harnett v. Harnett, 2014 ONSC 359, [2014] O.J. No. 237 (Ont. S.C.J.), at paras. 87-88)).
The Superior Court has recently clearly enunciated the law regarding the setting aside of domestic contracts in Harnett v. Harnett, supra,
90 A domestic contract will be set aside when a party was unable to protect his or herself. Such cases are generally predicated upon a finding that one party has preyed upon the other, or acted in a manner to deprive the other of the ability to understand the circumstances of the agreement.
91 The court is less likely to interfere when the party seeking to set aside the agreement is not the victim of the other, but rather his or her own failure to self-protect. The Ontario Court of Appeal in Mundinger v. Mundinger (1968), [1969] 1 O.R. 606 (Ont. C.A.) says that the court will step in to “protect him, not against his own folly or carelessness, but against his being taken advantage of by those in a position to do so because of their position.”
92 The court must look not at which party made the better bargain but rather, to whether one party took advantage of their ability to make a better bargain. In that taking of advantage is to be found the possibility of unconscionability. See Rosen v. Rosen (1994), 3 R.F.L. (4th) 267 (ONCA)
93 The test for unconscionability is not weighing the end result, but rather the taking advantage of any party due to the unequal positions of the parties. See Mundinger v. Mundinger (1968), [1969] 1 O.R. 606 (Ont. C.A.); Rosen v. Rosen (1994), 3 R.F.L. (4th) 267 (Ont. C.A.).
94 The onus is on the party seeking to set aside the domestic contract to demonstrate that at least one of the circumstances set out in subsection 56(4) has been met; then the court must determine whether the circumstances complained of justify the exercise of the court’s discretion in favour of setting aside the contract. It is a discretionary exercise. See LeVan v LeVan. 2008 CarswellOnt 2738, ONCA.”
S.(J.) v. S.(D.B.), 2016 ONSC 1704 (CanLII) at 27-29